False Negative HPT at 6-7 weeks?

[FreshLove]

Hi ladies,

I have 3 children (almost 6yrs, 3yrs, 16 months) and had the Paraguard (copper) IUD placed when my youngest was 6 weeks old. I check it regularly to make sure it is in place, and to track my fertility signs. Here is my question, have any of you been pregnant, but not gotten a positive pregnancy test? Or maybe gotten one really late (like 5 weeks past OVULATION).

I have not had my first postpartum period (my son still nurses a lot), but was SURE I ovulated around the 16-18th of July. The entire week, starting around the 9th or 10th, I was incredibly horny, like I would just be sitting by myself tingling all over, and all my husband had to do was give me a good morning hug and I was ready to go, lol. About halfway through the week (maybe July 15th) I noticed an increase in cervical mucus (again, I've had low cm due to nursing for months), not ewcm but definitely watery abundant. Then, by July 17th my pee smelled strong, every time I went to the bathroom I was like "woah!" I used to get this the week before my period. By Tuesday I was peeing every hour (another post ovulation sign that I typically get, but hadn't for a while because I have been nursing and no return of fertility, so I noticed the changes right away).

Anyway, I bought a pack of jumbo sanitary pads and figured AF would show up any day now. I have short luteal phases, typically 9-11 days, but when my period returned at 18 months postpartum with my 2nd child back in 2014, I had a 5 day luteal phase (not uncommon for nursing mamas). Well, it has been over 4 weeks since then, and like 6 negative HPTs, all with first morning urine. After the first two negatives, at what would have been 11dpo and 16dpo, I figured maybe I didn't O or it was later, and just continued to wait for AF. Then, the 1st of Aug, I start feeling little twinges and tugs in my uterus and by my belly button, along with round ligament pain. I sat there one day going Huh, the only time I have ever felt this is with each of my 3 pregnancies, weird. Take another test, and negative.

I wouldn't think anything of it normally, especially with the negative tests, except that the uterus twinges, round ligament pain, and now for the last week nerve pain in my hips/left leg and random nausea throughout the day have me feeling extremely pregnant. What gives!?!?

So, I keep telling myself it is IMPOSSIBLE to be pregnant on the IUD, and getting negative HPTs, but this feeling just won't go away.
Oh, and almost forgot, I called my ob/gyn Monday to request a beta blood test, said they would call back with my appointment time (they are busy). When I hear back from them, the lady says "Your first appointment is Aug 29th 8:15 for the ultrasound, and "take-in" appointment with the doctor is Aug. 31st, congratulations!!" I was so confused I couldn't say anything but oh, ok thanks. Weird!!!

Sorry for the long post, but please comment with any experiences!! TIA!

[FreshLove]

Oh, forgot to say, the IUD is in place, and I don't feel anything poking out.