please help me.

so, i only started my period halfway through freshman year (15). i had a very light but all over the place period. in march i went to go get a shot and the nurse mixed it up, and accidentally gave me the birth control shot. she said for three months it would be abnormal. it's coming to the end, and i've had my period twice this month and last month. i've never had a heavy flow,  nor a cramp. but today i did. i went to the bathroom and found blood all over and on my body. what's causing this? is it the birth control, since i don't want to have sex i didn't get another so maybe it's changing from not continuing? idk what's wrong with me?