Even if you doctor didn't find anything wrong, I would consult with another doc or gynecologist for sure. It sounds very much like muscles or whatever it is keeping the bucket ( bladder) from leaking have a weak spot.
A friend of mine had the same problem and she made me a drawing of how things work and where her weak spot was. Got it fixed and she was good to go without anymore ... wet panties ...

Those exercises might work in your case though.

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