This can happen to anyone: overdoing the exercising. It happened to me many years ago. Here's my story:
 
Things started off well. At an obese 185 pounds, I started taking aerobics classes twice a week. Eventually I changed to another gym that offered more classes and took a class offered 3 times a week. After that, I started taking step classes in additon to aerobics. My body's progress leveled off, prompting me to start weight training. Miraculously, I wound up losing 50 pounds over the span of 5 years with all these activities. The downside? I was miserable, tired, and my body started getting minor injuries because of all the punishment I was giving it. 
 
The moral of the story: exercise should feel like fun rather than a chore.

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