Confused.

I went to the doctor today to get the results back on my hormone levels. They all came back good but the thing is: i just eneded a 15 day long period, before that i didn't have my period since July. My periods have always been very long and painful. Not only that it hurts to have sex or wear a tampon. I was supposed to get my depo shot in February but i didn't get it. The doctor said it was because the depo shot but it has been this way before i got it. what could it be?