Pain after i use the bathrom?

sorry it sounds gross but im 14 years old and i just went to go poo and then when i was done my stomach hurt after i went but only for a few minutes then i feel gas in my stomach and with a little pain and i feel a little naseus , there was this one time when gas wouldnt come out so my mom took me too the emergency room and the docter gave me some milk of magnesium this happens occasionally but i dont know why it hurts please help me.