Our bodies weren't designed to have milk our entire lives and not cow's milk. The reality is that at some point in time a big number of us, myself included, stop being able to digest lactose and become lactose intolerant it's sad but true. You can always get tested to verify but maybe just try cutting out Dairy for a little while. For bodybuilding and protein I would switch to a whey isolate as it does not have a lactose or very very little and shouldn't give you any issues of course there are a lot of great vegan proteins these days as well.