Hmm - that's a tough situation. I wanted to answer this on the blog, because I'm curious if anyone else out there has had something like this happen to them. I've certainly heard of more senior animators changing the animation that someone junior may have done, or a lead or supervisor fixing up a shot late at night at the last minute (although most companies and animators in leadership roles certainly try to avoid this whenever possible, for obvious morale reasons), but if you mean that a fellow animator who isn't "above" you in the company randomly decided to change your animation overnight - well, that's just bizarre and plain old rude!
My advice would be to ask the animator in a very professional and polite way what happened, because most likely there is some misunderstanding going on, and more likely than not, someone probably asked him to make the change. If that isn't the case, though, then I don't think it's egotistical for you to follow up and find out what happened or why it was changed - even though you're doing commercial work, it's still your art someone is monkeying with, and they should have some kind of good reason to be fiddling with your stuff if they aren't supervising you.
This sort of situation is a good example that we can all learn from. If you're working at a company, then you are part of a TEAM, and you need to not only treat each other with professional courtesy and respect, but you need to treat the project and work with a bit of reverence. We're all pouring our hearts and sweat and souls into our shots, and we need to keep that in mind whenever we're giving feedback, constructive criticism, or deciding whether or not to change someone else's work without telling them. (Hint: don't do that unless you want to hurt feelings, anger people, and tank the morale of the whole team!)
Well, hopefully you're just stuck in a misunderstanding, but I think you're well within your rights to at least ask to be told when something in your shot is going to be changed by a peer. If it's someone in a supervisory role, though, then my advice would be to express to them that you wished you had had a chance to make the change yourself, but leave it at that. Sometimes being part of a team means being willing to let someone else finish your shot if it's for the good of the project, and if a supervisor decided he wanted to tweak your shot, he or she probably had a good reason for it that you just might not be aware of.