Please help-feeding- series problem 3 month old?

my son was born premature at 34 weeks he was tottaly healthy on similac advance first 2 days then milk until2.5months. i stopped cause my son was allergic to something in my diet(blood in his stool). went on alimentum which gave him green mud like poop almost after every feeding he wasnt sleeping during the day and was awake every 1.5h at night. we went back to similac advence tested for blood in a stool after a week no blood. he was a little fusy so we try similac sensitive which was great for 2 weeks when today i notice blood again. went to see adr she told me to start alimentum or nutramigen again. so i tried nutramigen for 4 feedings now he is crying and gy like never before so i just feed him with similac sensitive and now he is happy and sleeping. i have app with gastrologiest tomorrow. i dont know what to do. my baby is so happy on every formula except that special one like allimentum or nutramigen. i was just wonder maybe it is something different that is causing bleeding. i dont know any more there is not much blood. i called my dr waiting for her to call me back. any advice or stories. i am hopeless and just want to cry. i cant stand when my little one is hurting. help