Why is the entrance to my vagina swollen?

I've had this problem for about 15 years. It's not new and it doesn't come from having sex or my period or anything like that but sex or my period does make it worse.

Basically, the entrance to my vagina swells. If I put my finger in there, it's the "top" part of the vagina and it's swollen to the point of causing intercourse to be painful. I also have occasional itching in that area and a clear/milky discharge but with no odor.

I have been to the doctor and been examined countless times and they have never come up with a reason for this. I've tried everything from changing soaps to homeopathy and everything in between.

Anybody have any other ideas?