Not pregnant but contractions??

All day today, off and on, I keep having what feels like contractions, but instead of the long hard ache all over my waist like what contraction would usually feel like, it’s a sharp pain kinda in the middle of my lower stomach. It builds up like a contraction and holds for a few seconds and then releases and repeats, for a few minutes at a time and then it’ll stop all together for a while. I took a pregnancy test last week and it was negative so I don’t think I’m pregnant. Any idea what it could be?? I haven’t had a period in almost 2 months but I’ve taken multiple pregnancy tests with all negatives.