Thanks Nash and Karenee for replying.
Yup, Nash, you're right. It doesn't necessarily hurt, and I don't have to stick my finger inside. To a certain extent, it can be pleasurable to "tremble" this way. It is very near or on the G-spot. I do notice I get the best orgasms from stimulation near the front (clitorial stimulation?) rather than the back or deep inside the vagina.
I'm thinking it cld be the way I'm wired. Perhaps stimulation there impinges on the nerves that run down the leg. It's not a big problem at all, just a weird one, and I was wondering if any one experiences it. I know for example my husband doesn't have sore legs after sex, but I do.