Hi J.~
So I'm going to assume that you don't have a violent little girl, right? It's not about hitting, it's about investigating causal relationships. If I hit, then I'll get this reaction. So I think the best thing is to be totally and completely consistent, every time with every person.
I've always suffered a little bit of confusion about what the right words are to say in that situation. With my son, I realized that he started thinking "gentle" meant "hit" because he'd hit and I'd say, "No, be gentle." So I had to really work to reverse that, by using the word gentle while doing gentle touches. In my experiencce, it's better to use the words for what IS happening, not what you want to see happen. So I'd go with "No hitting" or "no owies" or something like that.
As for what to do, that's tough. People kept telling me just to quickly put my son down when he'd hit, but the problem with that was that he didn't care at all. That was fine by him. So I just kept putting him down, trying to ride it out as best I could. I think this is a pretty common problem at this age. Just model gentleness for her as best you can, *especially* when she's not being gentle with you.
It'll pass, it's just a crappy phase. Good luck!