Why Does One Bleed During Sit Ups?

Hi, I am 23 I bleed after doing some sit ups I am doing it every night sometimes I do it twice morning and night. I usually count the sit ups that I have. Just for instance just this morning I got 25 sit ups then at night I will make it 50. I make sure to have it like a lil bit higher counts. I don't have any idea why I bleed since I just have my period last January 4 so it was just 16 days. I am having my period regularly and I am counting and recording the day I got my period. It was all recorded I was just confused why I just have bleeding. I am already doing sit ups but I just stop it but while doing it or after doing sit ups I never did encounter such bleeding like this. I really need your answer for this. I was alarmed I don't know what to do. Thank you. Hope to hear from you.