hmm... a few things come to mind. first is you say you haven't exercised since last year, you might need to become more active. the other option is spiritual. it could be a form of a curse, or a spirit. personally i think it's physical activity, you might have slept wrong, but a spirit might be harming you. for instance, [something i was told as a child] it is believed during twilight [sunrise or sunset] spirits wander the earth and if you bump into one they might bite you, you can tell when this happens if you have a bruise you can't remember getting and it doesn't hurt.
i say try stretching, yoga or tai chi, if it persists go to a doctor. if other strange things seem to happen, then it could be spiritual, but i doubt it.