What Causes Weakness, Headaches And Dry Eyes?

Weakness in whole body when doing anything: eg if bending my legs, my legs go weak, if pegging washing out, my arms go weak etc etc. The symptoms last for months then go for 2 weeks then come back again. Having daily headaches and dry eyes. Pulling muscles in various parts of my back all the time. Had symptoms for a few years. Doctors are baffled!