Crushed after a race

I have experienced a few meets that I am absolutely crushed after I swim. In my first meet back after a 8yr "break" I swam a 100 Fr and was toast for the rest of the meet. Since that point 2 years ago, This seems to only happen in 200's or the 400 IM. Symptoms are totally drained, overheating, even light headed, for 20-40 minutes after I swim. Basically I am incapable of doing anything for this time span. I would assume it has to do with a level of fitness. But my question is how can I introduce training that will help me minimize this effect? FYI, I swim more or less 3-5x/wk, about 1- 1.5 miles over the course of an hour... I am 5'11" and a portly 230... Chris