Your baby gained 2 pounds in 2 months, so roughly 0.25 pounds per week, or 4 ounces. I thought the "normal" range was 0.5 to 1 ounce per day. Your baby has been gaining at a rate of 0.6 ounces per day.
I'm not a doctor or an LC, etc. But it does not sound to me like you have anything to worry about. I would do a weigh-feed-weigh, just to make sure.
The other thing I might do in your shoes is get a referral for a PEDIATRIC allergist and a GI doctor. Food allergies can be some pretty scary things and it sounds like your baby definitely has them.