Mothers are impossible, because unless you're lucky enough to get a nice, sensible one who realises that you are only human and won't always say or do the right thing... then they're looking for faults.
Spenceman's suggestion was good... make *sure* your boyfriend knows you're going to ask. Just say something like, "(insert name here), I get the feeling that I may have done something to upset you recently and maybe we haven't been getting along as well. Is there anything I can do to make up for it?"
Basically... grovelling in disguise. Don't give her any reason to get angrier. By dealing with it in a mature and honest way, you may find that she'll actually gain some respect for you.