Make sure you are eating a good balanced, nurtritional diet. If you don't know what that would consist of, look up nutrtion online and read up on it.
It woudn't be a bad idea to have a routine physical, if you can afford it. It is good for a person to have one yearly, and you can mention your symptoms. The doctor will just make sure everything is within normal ranges.
There are ways of altering the feel of your mattress if you cant afford a new one. You could put a piece of plywood under the mattress (between mattress and box springs), that will make it a lot firmer. Along with that you might consider a foam topper, the eggshell kind is cheap, or you can get the "space foam" variety, about 2 inches thick, for around $100. It is good support, but flexible enough to cradle your body a bit.
See to it also that you are getting plenty of exercise. A brisk walk in the evening after supper and two hours or so prior to retiring for the night would be excellent.
SOmeone else mentioned depression as a possibility, discuss this with your doctor.