Hi! I'm An 18 Year Old Female Who Has Had

Hi! I'm an 18 year old female who has had very regular, on time periods since I was 14 years old. I am on birth control. However, in the past few months my periods have changed drastically. I used to have 4-6 day periods with mid to low bleeding, nothing too heavy. Every period since August of 2015, however, has been incredibly odd. I get my period 4-5 days late, the day before and day of I usually have very intense cramps, it usually only lasts a day or less (sometimes I only bleed lightly for a few hours), and is accompanied by chunks. I saw my Gynecologist in October and she said that what I am experiencing is a uterine cast (which she says is normal). But the fact that my periods have changed along with these mid to large sized chunks coming out my vagina is so odd to me that it doesn't seem that it could be normal. I am really concerned about it this month because I had one large chunk and the rest of my bleeding has been a filmy white/red mixture. I took two pregnancy tests in November, both were negative.