What do you mean by sick exactly? Like sick to your stomach, flu like symptoms, etc.? And how often do you work out and for how long? Do you drink enough water to keep your body hydrated?
The first thought that popped into my head was the articles I was reading today about L-Glutamine. Its an amino acid that's produced naturally in the body (I think..don't quote me on the natural-in-the-body stuff).....mostly found in the muscles. Weight lifters use a supplement of this because after a rigorous workout, the muscles can be depleted by up to 50% of this AA. Taking it helps with recovery, less breakdown of the muscle, and other benefits.
My point is that I read that L-Glutamine also helps with the immune system and your resistance to get sick. I know from personal experience if you work out too hard and too often it leaves you open to get sick. That's probably the only reason I've gotten sick twice this year...too hard and too much. This is because a lack of L-G being in the body to help boost the immune system. My wonder, and its pure speculation, is if you are working yourself too hard then depleting yourself of L-G. That would leave you open to the chance to get sick.
Surely theres plenty of variables to the possible problem you're having. My was only a suggestion. If L-G is the problem, a big "if", then two suggestions and recommend the first. Don't exercise so hard and let your body recover more. Or decide to take a supplement for it.