The, um, traditional excuse might be "I'm tired". She can also say it honestly (which shouldn't be a big deal, but some girls aren't sure guys are okay with it), or find some other kind of excuse. There are apps for period-taking, that could enable you to know the red days in advance, if that suits you better.
Well she won't tells me the truth. I'm little bit experienced whenever she didn't acting like herself I know she's on her period. Yeah Im a golden guy no one claims me easily. My girlfriend should be happy to have me and blah blah