Does she say it to you when you ask for sex?
Do you just somehow understand? (How? Please write it down)
Does she tell you suddenly?
How do you get it?
Most Helpful Guy
Most Helpful Girl
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.0