Almost constant period pain between periods - scared

I’ve been noticing changes in my period over the past 2 years but over the last few months they’ve got worse. I’ve never had any kids and I’m definitely not pregnant.

I get period-like pains all the time and now the pain has spread to my lower back and legs. This is what period pain is usually like for me but it’s happening constantly and getting worse. Also my period used to last 7 days, now it’s usually 8-9 days and it’s a lot heavier now. They’re still pretty regular and I haven’t skipped one.

I saw a doctor about this last year and got an ultrasound. The ultrasound showed that my endometrium was too thick but nothing more was done about it. I asked about my abdominal pain and my GP insisted it was IBS. I don’t want to sound argumentative but I’ve had digestive problems almost all my life so I know the difference between gut and period pain. I was finally put on mefenamic acid; it’s helped the heavy bleeding and pain during periods but the pain between periods is getting worse.

I’m 6 stone, so I’m worried that being so thin has done something to my hormones. I’m afraid that it might be uterine cancer and I just keep getting brushed off because I’m 'too young’ to have that. I’ve heard that lower back pain is a symptom of advanced cancer so I’m really scared.

I’m afraid that it is cancer and I’m just being fobbed off and treated like a hypochondriac, and by the time someone does find it, it’ll be too late.