Hey there! Just wanted to let you know your Lo is not the only one doing this. I have a 5 months old baby boy who has been doing the same for the past 2 or 3 weeks.
My DS likes his milk warm, too. Try it, it might help.
As for the weight gain, don't worry. There are so many things that affect this. Mine, for example, does not gain any weight the week he has his shots.
Hang in there.
Monica