I went through this when I was teaching 6-8 aerobics classes a week, as well as working full time, with 2 little kids. I constantly had swollen glands, sinus infections, bronchitis, and my asthma reared its head. I was also super skinny, and not from trying to lose weight. Oh, and I also got up at 4:15 AM 2-3 days a week to teach at the gym.
After a bout with heart palpitations, from lack of sleep and too much caffeine trying to sustain this lifestyle, I decided to cut my schedule of classes. The issue slowly resolved itself, after years of this. It was just plain over training and lack of down time. While I ride and do other sports now, I am very careful about my rest time, both physically and mentally. Being 20 years older helps, too!
Everyone is different. I know I couldn't keep up the training schedules some others here do, even though I might want to. Get a good physical and maybe a nutritional check up.