TMI Alert!

So I have been on birth control for a year now and my doctor switched my prescription to a different birth control pill this month. Yesterday morning I went to the bathroom and when I went to wipe, I saw light pink spotting, I was a little confused because it’s never happened to me before and when I went to the bathroom later in the day, the light pink spotting turned a brown colour. During the day I was having cramps, but not like horrible cramps, just normal cramps. On last Friday, I had sex with my boyfriend and he finished inside me and I didn’t realise that, that was my most fertile time. I’m just curious if it’s the start of my period or if it’s something else. Any comments are welcomed. :)