It could mean a number of things. They could just not be in the mood. Some people just don’t need to have sex as much. It could be they are having a personal problem, which could be an infection if it’s a female and she doesn’t want to disclose it with you. They could also not be satisfied with the sex and don’t want to hurt your feelings by telling you. And the last thing it could be is that their’s someone else, but if nothing else is pointing you in that direction don’t assume they’re cheating on you. The best solution would be just to ask why.