Weird period

Okay so I started birth control in the middle of my cycle (my doctor said it was fine) and I just started my period today (during my sugar pill week) but when I went to change my tampon there was this weird sort of discharge or something in my panties. It was almost like fatty looking and very odd. I've never had anything like that happen before, has this happened to anyone else? Does anyone think it might be because of my birth control? Maybe it prolonged my period too long and my lining was just really thick? Help!!