Is Cramps And Lower Back Pain Sign Of Endometriosis?

I get cramps and lower back pain every month about 7-10 days before my period starts. They are not severe pain and do not require any pain killers, but the back pain is like a dull ache and it's very annoying. I don't know if this is a sign of endometrisis but all pelvic ultrasound and HSG is clear, and the doctors never mentioned anything. I've been trying to get pregnant for 15 months, so not sure if endometrisis is causing the dull ache 7-10 days before period as well as infertility. I don't get pain any other time during the month except having a different feeling during ovulation.

Should I worry about endometrisis?