I think I'm a tomboy. Is it a problem?

You should always be yourself no matter what other people say. If you enjoy hanging out with the guys and they're OK with it, why not? If your girlfriends have a problem with it, that's their issue, not yours. Maybe you can help them open up their minds a little and invite them to hang out with the guys too. The guys would probably appreciate it and the girls may find there's another perspective to the way they think.