Oh boy... This won't be an easy fix.
Let's see, so you have a female friend, let's call her Alice.
You also have a girlfriend, let's call her Kate.
I've been here before. Oh man, it's all coming back to me now. Right, so the most important thing to remember is the chronology and social status. Are both from the group of 'shy girls'? If so, then you are in for misery if you're not supremely careful and perceptive. You've got to listen and do exactly as I tell you. I've lived this nightmare and made every mistake.
If you met Alice first, then her friends, and then got romantic with Kate you are most likely treading all over Alice's heart. These girls can hold their feelings in for years, and you may have had Alice at 'hello'. Unfortunately you are in a corner right now. You are on good terms with Kate, so there's no sense in throwing that away to be with Alice. When one of the other girls in the group is alone, strike up a conversation and ask her why Alice is being so distant. Once you're sure of the situation, either tell Alice that you aren't interested in being with her and that it's not her fault- that you're just a goofball and didn't want to hurt anyone but messed up and did. Or, if she's just being distant, be nice to her and don't mention anything.
Best of luck!
-Penguin
