Why would my two year old daughter try to choke me?

My two year old daughter is usually the most loving toddler you could imagine. She actually is so well behaved 95% of the time, that she makes me wonder why there's a phrase called "terrible twos". She's been doing something that alarms me, however. She will ask to sit in my lap facing me and I'll be talking to her and giving kisses, etc and all of a sudden (she's done this a few times) she will place her hand around my throat and slightly squeeze. She'll say "I choke you, mamma". It really freaks me out and I of course tell her not to do that; it isn't nice. She kindof laughs and then acts normal again. I few times she hasn't stopped reaching for my neck and I have to set her down. She's at home with me 24/7. She's never been in anyone else's care, and I know there's no way she could have seen something like that on tv. We don't watch adult shows with our kids in the room ( our son is 1yo). Why is she doing this? Should I be concerned?