I would not react the way she reacted but you must understand it from her POV. She only has your word that nothing happened, and she would like to think that you, in bed and asleep, is her terriatory and hers alone. Being in bed with someone and its only a small leap to kissing or sexual behavior. Does she know this girl? If it is someone who she knows of and knows thatyou are really close to, you may be able to ask her how she would feel if it was her sleeping next to a close friend and how you don't think gender should matter and that you would trust her so she should trust you. You need to understand that women are very needy and terriatorial. You are HER man so she does not want other girls ANYWHERE near you.
But the more you push, the more it will look like it matters to you and she will assume you want sex or something. Juts tell her you think she overreacted, but if it means that much to her you won't do it again.
Whats more important, a little comfort with the friends or keeping your woman sweet?