I (M23) got her (F29) consent to ask about this here. And it's not a question about if she's lying or something like that. I know she is 100% honest about everything here:
For starters, it's not pain, or that she's not satisfied. She's always wet and want me to penetrate her. It doesn't matter if she cums before penetration or not, and it happens with PIV, fingers or toy. She enjoys penetration a lot, and it's usually her favorite part. But about a week before her period starts, during her period, and a few days after, she cries when I'm "done" penetrating. During penetration she enjoys it and it just feels good, but when I take out whatever is inside her she cries. It's not happy or relief tears, and it's not sad tears either really. She just cries for a minute, and then is a bit off for a few more minutes. Then she's fine.
She has trauma, but nothing sexual. Trauma was my first thought, but she's sure it's not that.
She simply doesn't like crying (understandably) even if it's not sad tears, so we don't do any penetration at all during this period. But both of us would like to "fix" it.
Any ideas? Medical issues would make sense if it was all the time.