I have recently been put on birth control and I am getting terrible cramps coupled with light spotting?

I went to my gyno for the first time and my mom had informed my doctor that my period was abnormal to the point where sometimes I wouldn't get it at all for a month or it would come weeks late or I'd even get it 2 times in one month so he put me on birth control...I've been taking it for about 3 weeks now and 2 or 3 days ago I got what I thought was my period but it was different its barely even there and it didnt come with the regular symptoms of my period like tiredness bloating or cravings nothing it was just light bleeding but yesterday all of a sudden my stomach started cramping terribly to the point where I had trouble sleeping last night and has not subsided yet....I was just curious if anyone knew what is happening to my body my friend told me its just my regular period my mom told me its my body adjusting to my birth control but all ik is its extremely painful any advice anyone has on this would be great :) thanks!