Alright, the first four times you couldn't feel anything, so what happened? Did you just sense something? Or did you only get the feeling in your vagina but not on your legs?
And did it feel like there was an object in your vagina or was it just pressure?
This is going to sound just as crazy as your question, but wth... Do you think you have a ghost?
That's all I can think of... I really need a little more info. So until then i'm gonna go ahead and say yes, this is something you should be worried about.
Please though, give us some more information about what happened. This is very intriguing.