Here's an odd one; odd due to a specific and repeatable symptom.
For starters, I'm 37, slim, male, rather good health, rather good diet, only vice is moderate smoking (which I plan to drop in the next few weeks for reasons that shall become apparent). A while ago I started experiencing what I'd call my "Mystery Pain", which turned out to be gallstones (which is really weird, since I don't fit any profile for a "gallstoner". As far as we can tell, I just randomly won the Minor Abdominal Disorder Lottery) when I had an ultrasound a month ago. Around February was when I started to suspect, "Maybe this isn't just indigestion?" due to various odd side-effect symptoms kicking in in lieu of the mystery pain.
One odd side-effect symptom that started showing up was, on the schedule of the gallstone pains (IE: only at night, give or take a few hours), as if I'd start to go into shock. Obviously, I didn't go fully into shock, but it fit all the symptoms of a person starting to head that way (EG: suddenly feeling cold and shivering, even when the temperature was fine, limbs going cold while my head and torso either remained warm or warmed up, feelings of dread and "Something horrible is happening!" such like for no good reason, et cetera) before things returned to normal in an hour or so. Both my primary care physician and the Gastrointestinal specialist I've spoken with are fairly sure that this was my body noticing something "weird" going on (namely, a stone being thrown painlessly) and sounding the "all hands on deck" general alarm.
The other odd side-effect symptom is the one I'm actually asking about. The best way I can describe it is like a "bronchitis around my heart" that waxes and wanes; it might fade in one day, linger around for a few days (or just one day), then fade out another day, remaining away for a few days or a few weeks before fading in again. My heart doesn't actually hurt or anything, but I'm just more aware of my heart beating and it feels like a basic pulse/pressure test should turn up something strange, but nothing crops up; pulse is bordering on low, if anything, and the blood pressure not only isn't unusually high nor low, but even the difference between systolic and diastolic isn't remarkably wide nor narrow when compared to my checking it while feeling fine (or comparing it to any chart). Occasionally there's the rare scary "twitch", but it doesn't feel like a mis-beat of my heart, as much as it feels like a muscle near my heart twitching in a little casual spasm. This notion of being something neighboring my heart and not actually my heart itself is backed up by the fact that a cold stethoscope or a sudden hand on my chest seems to cease the spasm (as is the way with innocent muscle twitches). There's never any actual pain-pain, with the closest to discomfort being the aforementioned twitches, which aren't any worse than if I were casually accidentally leaning against a doorknob (minus the pressure, since it's more an odd little sensation than any actual bad sensation), or the discomfort of a cough.
Here's the odd repeatable part, though. If I'm in the middle of a bout of the "bronchitis around my heart" thing, usually doing something laborious doesn't make it any more noticeable, unless I'm lying down. I discovered this when I'd gone to bed early and had a pile of books with me. I decided to not sit up to move the books but just sort them while lying down, and noticed that I again had the feeling like my heart was beating far too laboriously or far too quickly or far too.. anything bad! Alarmed, I sat up thinking, "Great, so I have to get dressed, put on shoes, go downstairs, call 911, and.." but then it went away after sitting up for a panicked minute or so. In testing it, hoisting books around while sitting up was "safe" while hoisting books while lying down was "alarming" (although the alarm could be cleared up by just sitting upright again). The same goes if I trot up the stairs two at a time and hop right into bed -- if I lay down, it'll feel like my heart is trying to burst from my chest, but if I sit up in bed (or don't run up the stairs like a madman), it's nothing unusual.
At one point, early after gallstones were suspected but before the ultrasound found them, I had a visit to the ER due to worrying I was having the start of a heart attack on my way to work (Very aware of my heartrate, suddenly feeling dizzy and thirsty, and an annoying cramp in my left arm). Hitting the ER, they ran an EKG which was -almost- normal, except for a "twitchy bit" near the end. The "twitchy bit" prompted blood-testing and a pair of chest X-Rays, but nothing showed up on either the test nor the X-Ray and I was sent home with a bill of "atypical pain" (not even angina). Based on those simple tests and all, my primary physician and I are leaning towards this odd heart-bit either being some minor passing infection/inflammation under my pectoral muscle and/or general anxiety about the gallbladder details (at the time, worrying about if it'd be worse than gallstones, at this time, worrying about the gallbladder removal surgery). I'm putting off the smoking cessation until post-surgery, since if it's pre-surgery anxiety attacks, I think I'd rather have a couple weeks of extra smoke than a couple weeks of extra panicking.
Still, that "effort while horizontal vs while vertical" variable is quite odd. It seems like it should be a clear indicator of something, but I've no clue what that something is. Attempting E-Research into it isn't finding anything that fits, the closest being some fluid build-up affair that's really not sounding like it (due to my breathing having nothing to do with it, as neither a cause nor an effect). Ring any bells for anyone?