where's the line for dubcon?

i'm writing a fic between two characters who are reconnecting after some time. they have an unspoken agreement where they're not talking about how they've hurt each other in the past. after reconnecting, they become friends with benefits. character a, deep down, is still angry with character b. during one sexual encounter, after character b orgasms, character a keeps going, purposely overstimulating b. both of these characters care about each other and character a isn't trying to do something evil. character a is just overrun by emotion, and a momentary desire for revenge of some sort.

the overstimulation isn't something discussed, and when b begs a to stop, a feels guilty and almost embarrassed.

i'm wondering if i would need to tag this chapter for dubcon, or if i would just flag it elsehow in the notes? but even then, what would i say?