How do I express displeasure to my girlfriend when she makes fun of me for certain things?

I’ve been going out with this girl for about 4 or 5 months and overall i really like the way things are going. She’s the type of girl I rarely meet and she’s very into me so a lot of the times I’m really happy.

The problem is this: she likes to make fun of me a lot. Usually, I don’t mind, but she doesn’t know yet when she hits my pressure points. There’s a few things I’m very sensitive about (like anyone I suppose) and she sometimes picks on those things. When she does, it sticks with me for a long time and keeps coming back to me.

Now, normally, I think the solution to this problem would be: “just tell her you don’t like it.” But there are two reasons I hesitate to do that:
1) I did it once before and she felt really bad. I hate making her feel bad. Don’t get me wrong, it worked (in that she stopped bringing it up) but I ended up feeling bad about what I said and back peddled a bit immediately.
2) She likes the fact that I’m manly. So far, since I haven’t told her too many of the things she does that I’m unhappy with, I’m not sure if she considers showing emotion (or a lot of emotion) unmanly. Maybe people can tell me if I’m over thinking this part? Am I being stupid on this? I’ve been a “nice guy” my entire life so I never really focused on being manly till recently when I realized women like it. I mention that to point out that I’m not sure what women consider “unmanly.”

P.S. We are in our mid to late 20’s. It’s entirely possible that this question shows us to be emotionally immature, but oh well, that’s where we are.