Junel Fe and period

So I've been on junel fe 1/20 for about 6 months and the first three months went well. For the past two months I haven't gotten a full on period the way I normally do. I get very very light blood (not enough for a light tampon) and then very dark brown blood (which I know is old blood). It never lasts more than 2 days and even the dark blood is barely enough for a light tampon. Anyway, I've been sexually active but I've taken many pregnancy tests so I know I'm not pregnant, I'm just curious as to why I was fine for about 4 months then all of a sudden this starts happening. Any suggestions?!