Rather than assuming, ask her. She's the only one that truly knows.
how long have you been together? how often do you see her? how often do you go out to dinner?
I was that time of the month for her?? Well its no wonder she cancelled. Maybe you should have said, "I'll come over with a nice bucket of rich chocolate ice cream and watch a movie with you. How does that sound?" Then bring her some flowers and rub her feet. THAT is what she indeed needed. You would have gotten a bj out of that for sure.
Sounds like she is pretty bored with either you in general or your lack of skills between the sheets or both.
p.s. we talked today and everything seems fine