Hi there, thank you for posting your question. My answer is not intended as a substitute to visiting your own doctor but I hope I can be of some little help.
There is probably nothing seriously the matter with you. It just might be your age. Relax and do only the things that feel good to you. Post again in a while if things are not working out and give more details.
.
Dr Andrew Rynne.
www.andrewrynne.com