12 day period?

I've been on my period for 12 days for some reason? Ive been on the pill for 3 months could this be why? Some days its a super super light where I could wear one panty liner a day and it be a light red mixed with brown and then it gets a little less light but not heavy by any means I would have to wear a super thin pad and it'll be a little darker with little clots in it at one point I thought it had completely stopped because I hadn't bled in a day I didn't even need a liner but after that i started slowly again what could be this issue??