Moaning from a back massage?

I spent the weekend with a friend and met up with an old female friend of ours. Eventually we ended up in front of the TV and my friend asked her to give him a massage and she did. I have some experience so I gave her a few tips and offered her a back massage. She accepted and was moaning a lot and I think a bit more than normal. Anyway she is a extrovert so I'm not sure what to make of this. I am an introvert and I'm actually terrified of girls in general and I just make a fool of myself. A answer would be really helpful.
Moaning from a back massage?
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