What does it mean when you have been dating or seeing someone for a little over 5 months exclusively and then they tell you that you are "Just dating"

It means you aren't exclusive. And you aren't. It also means "back off" if you've been pressing on any particular issue(s) recently.

You should back off and start opening other dating avenues. She's basically telling you that you two aren't serious and that usually means other(s) have already piquéd her interest.

It basically means there's no commitment

I wouldn't assume it means any of those things. It could mean, "don't tell me what to do; we aren't married." I think you need to give us more context. More important, ask HER what she meant. Don't ask us.