I got abdominal pain for years. It seems to that the pain is in relation with my period. The pain starts couple days before. I also got heavy periods and lay in bed for days.
PID causes pain all the time, not just with cycles.
If you're concerned about STIs, just get tested. You should also talk to your doctor about these pains you're having. While some cramping with periods is normal, it is NOT normal to have severe pain that leaves you in bed for days.
It's not an STD - you've tested neg. Test again if you need to - it doesn't hurt to get tested. But definitely talk to your doc about your pain. What you are experiencing is not normal. I had severe endometriosis and what you describe sounds a lot more like that than an STD.