Butt pinching is inappropriate. I think she thinks it's fine to be inappropriate to you, but she doesn't like it when you do it to her (even though unintentionally.) She probably thought you were coming onto her, rather than making a simple comment. (I assume she really did smell good, and you meant nothing else by the comment than that. But an NT would read all sorts of weird into it.)
I read somewhere on this site that aspies are often the victims of intentional picking, because we don't pick up more subtle cues. So, we're more likely to have our butts pinched as a joke, or be poked, or have our hair pulled, whatever, since our reactions are different it's easier to laugh at us, and we don't realise it's happening (or if we do, we're not sure why.) In my case people didn't tend to do the butt pinch, tap on the shoulder thing, because I'd "whale" on them, but my son often gets poked, prodded, teased, because he reacts differently to NTs, and the kids think its funny. Part of me wishes he'd over react (as I used to) rather than "Spock out", because at least over reacting put a stop to it.
I would suggest that your friend was being inappropriate to you, and didn't expect you to realise it. She wasn't meaning to be "mean", but she was getting a chuckle out of it. When she thought you were doing the same to her, it freaked her out. And to be honest, it serves her right.