I Am Tired All The Time. I Have Gained Weight

I am tired all the time. I have gained weight (20 pounds in 4 months). I exercise 5 days a week, treadmill, elliptical and other exercise machines. I follow the south beach diet eating plan. I am 62 years old and have been active all my life, ride horses, move hay daily, etc. I recently had extensive blood work done to check my endocrine check which showed that system is working correctly. I am at a lost of what to do. I know that excess weight is very concerning and can potentially cause serious problems. Do you have any suggestions?