Severe lower back pain, nausea and now a 2 week early period

[Megzouat]

Hi all. So I'm 26 now and started my period when I was 12. I've always had various issues with my periods. From really painful and heavy Ones to extra ligh ones which last for weeks. Around 4 years ago my periods stopped altogether for 14 months and last year they stopped for 8 months (this coincided with my sister's pregnancy so I assumed it was some weird  hormone related thing) . For the past 12 months my periods have been regular and have lasted 5-8 days but are less heavy than usual. The only problem is that I sometimes get crippling back pain in my lower back which can't be relieved even by strong painkillers about a week before and it just disappears when my period starts. I had my last period from 6th-11th Feb but woke up 4 days ago with intense back pain and then yesterday 27th, I started bleeding really heavily and getting really bad cramps.
I'm not sexually active so don't take any contraceptive meds, I've been tested for PCOS and endometriosis. I'm reluctant to go to the doctor as I don't like being touched down there but am worried I might have to if I keep getting unexplained crippling pain and periods that start whenever they feel like it. Does anyone have any ideas what might cause this?