Oh I understand your feeling.depends on how are you feeling, if you are so unsure and sad you should not go but if it will be fun and you can deal with the situation, then go.I do not know If you have a dermatologist and what treatment you are doing but seek the best treatments.
I'm sure your friend will understand either way, if she's really your friend. If you're not feeling up for it, having cramps, etc., then that's a good reason to postpone until you're feeling better. On the other hand, having a good girlfriend night might take your mind off things and really cheer you up. Bottom line: never let your skin keep you from doing anything fun!!!!
It can definitely be frightening - I often skipped out on sleepovers all the time because I was afraid of them seeing my acne, but trust me, it's better just to go! Getting used to going bare can really be terrifying, but trust that your friend will be kind about it.
A lot of people actually don't draw attention to your skin, and you'll find that you'll become ten times more comfortable around your friend because you don't have to struggle with make up just to hang out with her!