My 4 Month Old Is On Similac Alimentum And Was

My 4 month old is on similac alimentum and was taking 6 oz every 4 hrs the past 3 day she has barely been eating 3 oz at a feeding she has always spit up after every feeding and she is still spitting up when not taking as much she is on Zantac 3x a day .9 this doesn't seem to help spitting up. She was 6lbs 7oz at birth and now weighs 9lbs. She is in the 3 percentile per her doctor about an hr after each feeding she spits up a ton of clear liquid and her doc doesn't seem concerned but I am and I don't know what to do when she was I believe 3 weeks old they did a test and found blood in her rectum she was constapated at the time so I guess they assumed that's why but they have never checked again to make sure it's gone it like they forgot about it