of course sex can cause muscle pains! especially if you do it rough!

it could be teh positions you get into, or the length of time you were getting busy for. could be numerous amount of things, try not having sex for a couple of days, and see how you feel then.

if its just a mild soreness then its nothing to worry about (i actulally enjoy the soreness, it remionds me of what we did the night before, and turns me on!! ) if its more like serious tiredness, or pains you should see a doctor.