GQ, I agree with much of the advice about addiction and/or mental issues. However, I think other than BM, everyone has pretty much overlooked the obvious. The physical possibilities.
Yes, you are an amateur body builder. Probably in great shape. Google the name Hank Gathers. He played basketball at the University of Loyola Marymount about 15 to 20 years ago. ULM had an INCREDIBLY up-tempo style of playing. They scored buckets of points. He was 20 or 21 when he dropped dead at the free throw line from an undiagnosed heart condition. He played fast tempo basketball every day of his life and had to be in fantastic cardio shape. And he just died. Very, Very sad, but it happens.
My first advice is go see your doctor and have a totally complete physical. Every test your doctor can think of. Then maybe go see a cardiologist to re-run the heart and cardio tests. Then go see an endocrinologist to have him or her run tests for diabetes, pancreas, thyroid, etc. Finally, see a urologist for any other physical issues with your performance.
Once the physical has been completely eliminated, then worry about possible mental issues.
Good luck with your situation.