You’re Roleplaying
If you and your partner are doing a lot of intense power play and roleplaying, tears may simply be a sign of getting into character. Being dominated can sometimes include begging your partner to do something, pleading with them not to do something, pretending you need to be punished or taking your punishment. Tears can be a very normal part of the game. If you feel fine emotionally, or if it feels like you’re “in character” while you’re crying, you probably don’t have anything to worry about.
You’re Happy
Plenty of women cries happy tears during s*x. You might feel grateful that you feel safe enough with your partner to explore powerplay. You might feel relieved that you’re permitting yourself to go to places that you may have not considered before. Or you might simply feel a lot of love for your partner at that moment. If that’s the case, let yourself cry! Explain to your partner that they’re good tears. If he has a hard time believing you at the moment it might help him to hear you explain it after the fact.