Will lap swimming cause swollen breasts?

Doubtful. I have never heard of this happening. I cannot determine how this might happen. If anything it may firm the muscles under the breasts and promote a more well supported breast. Good Luck.
Swimming won't. Any style of swimming won't cause your breasts to swell. If concerned, see your PCP for consultation and tests.
No. I wish it did, but I have never seen it. Saorry if you think this is a wise-guy answer.