I've had blood work taking so hopefully it will show up on that if it's something like that. I hope not though. What you went through is scary.
They checked my bp while I was in hospital a few times a day and it was always normal. It's never been checked when I faint though.
I forgot to say that they checked my heart rate a few times every day too and it was always over 100 usually around the 111 mark that isn't normal is it? They'd say "that's a big high" but they wouldn't mention it again.
I had similar symptoms (except for constipation) and was diagnosed with pyleonephritis (kidney infection). I was 27 or so, and it started with a urinary tract ifection that I ignored because I could not miss work. It took a week in the hospital, and more fainting and a second trip to the hospital before I was diagnosed. I had fainted so much, vomiting, tired, etc... By the time it was diagnosed correctly, I was very sick. After the second week in the hospital, my veins were no good and I almost did not recover. Because of that, I was told that I am more likely to have recurring bouts of it. It took a long time to recover. It seems to be a hard thing to diagnose, and if it happens to me again, I will request bloodwork to be sure it is gone. It seems like I get it more often now, 20 years later.
If you still have the dizziness, it may be a middle ear problem.
For the constipation, start a probiotic and consider Metamucil and magnesium citrate tablets.