lower middle back pain while swimming

I noticed while training for IMLP that my lower back is sore. I got some ART at Placid and it got better and now I'm back in training and it's sore again. I'm thinking I need some more core strenght and I'm going to start some exercise to improve that. Do you think it might be something else? technique maybe. I'm a prettry good swimmer but the pain seem's recent. I don't know why really.

Nathalie