It could be that your body isn't used to the type of exercise you're doing. Or you may be overdoing it. If you're doing a heavy weights session every day, STOP! Either do a full body routine every other day, or if you have to exercise every day do a split body routine. I'd recommend you take some Glutamine, works a treat for aching muscles. Think it's not too pricey for a bottle of tabs in Holland and Barratts