TMI (sorry!) Period Problems

So I just started birth control pills this month after not having a period for a little over 6 months. I went to the doctor and she said it was normal for me to have them irregular and I was tested for PCOS. My results came back negative (thankfully because PCOS is such a big fear of mine especially because I'm barely 16.) However i just started my period last night and had some unusual clotting. I expected it to be brown because of old blood however it was more pinkish and red and white mixed (sorry I know this is gross) but I just want some sort of explanation. I've only had clots a few times but they're quite small throughout the day but this one was quite large, around the size of a bottle cap. I heard it could be tissue but it was quite a bit of it. I'm just concerned.