Obviously I can only guess at what’s going on, but while you say that you have a “sturdy” relationship, perhaps it’s not as sturdy as you think. It appears that there’s a lot of anger between the two of you, and you manage to keep it bottled up most of the time, but after sex you let your defenses down and the rage pours out. One or both of you are frustrated at being in this relationship, and it shows at a particular point but I don’t believe it’s isolated. My advice would be for the two of you to see a marriage counselor. Maybe theses sessions will show that I’m wrong and that there’s nothing to all of this, and that would be great. But if the counselor discovers some underlying problems, then better to deal with them now than to let them fester because at some point, the damage would be irreparable.