Cause of haywire periods?

[SecretMongoose]

My periods have always been completely and totally regular, 28 days bang on.
Over the last year they've been increasingly random, varying from 21 days to 35 days. Plus sometimes they are barely more than spotting, sometimes normal, sometimes heavy.
I've been to the GP and had blood tested a couple of times for suspected menopause but it always come back normal. Had an ultrasound, also normal.
GPs don't seem particularly interested, but I kind of think there must be a reason behind it? Why would they suddenly go haywire? Any ideas?

Early 40s.

I may have other symptoms but have a few other issues so it's hard to know what they are down to. I have regular joint pains, disturbed sleep, tiredness but have a couple of autoimmune diseases that also cause this sort of thing. Nothing typically menopausal, like hot flushes.
It's a huge pain in the arse because I never know when I'm going to be on my period!