Help? Please

I have a really weird thing that happens to me every month around when my period starts and it's been happening for a few months. It'll start with pms (headaches, cramping like crazy, and horrible back pain) for about a week and it'll get worse until the day of my period, and it'll start as spotting and I'll bleed really really light and barely need a light tampon, and it lasts 3 to 4 days but towards the end of my period the blood with be a dark reddish brown and I'll have cramps for a day or two afterwards. But I used to have really heavy, six to 7 day periods and it's worrying me a bit. Any advice? Anything is appreciated. Sorry this was so long lol