Wonky periods tmi

I've noticed over the several months my period has been changing. Normally I am very consistent every 28-30 with bad cramping the day before, I'd bleed for 5 and spot like a day or two, totalling 6-7 days. I've noticed that they've become closer together, 15-20 days apart, starting with near crippling back pain and I would basically bleed or spot heavily for 9-11 days. Heavy is normal for me.

I'm 31 and I haven't started or stopped any medication. I'm not on b/c due to being fixed nearly 3 years ago. I've tested and get neg or not pregnant. My diet/routine hasn't changes. Any ideas why this is happening? Or how I can go back to my norm?