i've had intermittant pains throughout my pregnancy & ive found swimming extremely helpful - just being in the water takes off a huge amount of pressure.
Tuesday of this week i had pains in my lower legs - i took myself to the local pool to see if it would help before stressing myself out getting to see my midwife & my pains were gone when i got out :).
Just Something to help if this turns out to be "just growing pains" which is what all of mine have been!
It may be from a pinched nerve or something, but you should definitely get it checked out. Pain is the body's way of telling you something isn't right. You'd hate for it to be something life threatening, like a blood clot or something.