I have been thawing frozen milk for bottles...?

It smelled funny lastnight.And the past few days he has had horrible gas where he screams all day and night. Is it possible the milk is bad? Its from mid Feb. so its not that old. Any ideas? We have tried everything to get him to relieve the gas. He had diarrhea yesterday was well. I feel awful about this. I wont give him anymore until we figure out the culprit. I have a feeling my entire 200 oz. freezer stash is bad...? And my son just turned 5 months old yesterday so he is not a newborn with this gas problem.