What Causes Back Pain After Stopping Birth Control Pills?

Hi, i'm 25 years old and have had previous POS problems. i recently just got on the birth control pill (Diane 35) for a month. it didn't suit me so i left it and got my period in a few days and didn't take it again. a week after my period finished, i had to the morning after pill. a week following that i got a horrible back pain on the lower right side and the morning after i got my period again. i thought it was because of the pill. i also felt like i needed to urinate when i didn't and every time i would go, very little would come out but it didn't sting. my period finished after 3 days but and i took dissolvable cycstitic relief medication for 2 days, that problem is better but the pain in my right side still persists. why is that so?