I get this all the time in my right ear…like not even when I’m working out. I’ve tried some meds through my doctor none of which helped. I informally consulted an ENT colleague of mine (I’m a physician myself) and he said it’s only a problem if it annoys you.
It’s due to the valve like function at the end of the tube not working properly and the fluid not draining out of the ear effectively. Which is why it sounds like…wait for it…water in the ear 
Why it would only happen during exercise is beyond me…