So my period ended 5 days ago.Today,during intercourse I bled/but just a little/ and I'm not a virgin.After a few hours I began experiencing sharp lower abdominal pain.Also I feel as I'm about to pass out,feel the need to vomit as well.I have low blood pressure.
In the last two years I have had two cases of abnormal bleeding/between periods/ .I thought it was all due to stress,but now I'm getting worried.Any ideas what might it be?

There are many possibilities, and it's impossible to predict what ultrasound will show. That would be the usual first test. Unfortunately some of them don't reveal themselves right away, and you just have to wait and do some research (PID, fibroids, endometriosis are some).