Example, PID is in the lower abdomen/pelvis area. Endometriosis can cause pelvic pain as well. But you don't mention anything about irregular or heavy periods, generally this would give back pain too.
I would see your doctor, this could indicate an ovarian cyst as well. Since you have been on the pill so long, it may be that the pill is not working for you either, a blood test can confirm your hormone levels etc. If the pain becomes more severe prior to your period this is when you need to see your doctor so you can get an ultra sound done which will give a clearer picture of what is going on.
There are too many "What could be" to pin point it without some tests at this point.
Hope this helps Hun.