Sorry for tmi

I'm on the pill and have been for 8 months and I have had my period for three weeks then it went away and now it's been back again for 4 so far, it was really light and I'd only change my tampon twice a day and wear a pad at night, I went to the doctor yesterday and changed my pill which I started today, at lunchtime today I had a big blood clot come out and I've never had blood clots before it was that big I actually felt it, now in changing my tampon every 3-4 hrs which isn't normal for me, has this happened to anyone I'm really worried and don't know if this is considered normal I have really bad cramps at the moment too