try changing your diet take some iron pills with vitamin c add a little more red meat to your diet and try swimming for exercise that will help with the joints as there is no body pressure in the water. drink lots of water and see the doctor about arthritus ( sorry cant spell that word)
it could just be through sheer tiredness and worry that this is happening to you,try to relax more then maybe the muscle pain will ease and the sex will gradually go back to normal once you sort this out,good luck.