Anyone get headaches when lake swimming?

I get these awful headaches when lake swimming. I am training for a triathlon this summer and it may be my nerves. I have been swimming in a pool all winter and the lake does bring a bit of the unknown (in more ways than one)... but seriously. At the pool, I do think I have been cheating more than I had realized. I breathe a lot at each end when I turn. In the lake, I have to keep going and going and going and I wonder if that has anything to do with it. Yesterday, when I got out of the water after only swimming for about 20 minutes I thought I was going to throw up! My head was throbbing...I was so nauseous! ...the lake wasn't all that cold, but it was a tad bit chiller than the pool. Any ideas?