It sounds like round ligament pain. I swam during both of my pregnancies, and I always had to take the first laps really slooooow because I would get a pinching, tugging pain on one side or the other (Charlie horse was a great description). I couldn't avoid it - I just had to go slow and work through it and it would go away. It was the worst about midway through the second trimester, but I think it can start much earlier that that. Hope you're feeling better.