Cramps While Not On Period

For the past few months, I've been experiencing mild cramps when I'm not on my period. I went to my doctor about it, I got an ultrasound to look for anything out of the ordinary but everything seemed normal. I'm definitely not pregnant because my periods have been the same as they always are (on the pill, at least)

I'm also wondering if I could have endometriosis because I have severe cramps when not on BC. Could that be a cause, and can it show up out of nowhere? It could also be IBS but usually I can tell the difference between menstrual cramps and intestinal cramps.

I was curious if anyone has ever experienced something similar to this and if they figured out why. Because at this point I don't have any possible answers to why I'm expecting it. Its rather frustrating.