What Causes Missed Periods While On Depo Shot?

hi im 19 i had a depo shot at 18 and bleed for 24 days stright and was in extream pain i went to the hospital and they said i was having a reaction so when i went to go and see about changing my birthcontrol they said it was fine gave me the shot again and told me to take the pill but not t take the one that would give me my period i have mved since then and still have not had a normal period so i thought i could be pregant after taking several test they have all came back negative so last saturday i started taking the birthcontrol pill again to see if it would help give me a normal period again but earleir today i became sick to my stomach thinking i needed to use the restroom so i tried but nothing happened and just a few minutes later i began vomiting and im stilll in alot of pain so i figerd okay ill try and go to the bathroom again wich this time i got to use the bathroom for a bref moment but ive tryed laying down changing my clothes curling up in a fetal position and nothing is helping with getting ride of the pain and as the pain gets wores the more i want to vomit