Guys get this thing called epididymal hypertension which is what happens when they get a lot of blood flow to their sex organs when aroused. From this they can get discomfort, pain, heaviness, and aching. Orgasm relieves it but otherwise, that's when a cold shower comes in handy. Even distracting yourself until the erection is gone.
Very well could be the same for women. You probably have a rush of blood flow to your vagina and all related areas.
Is sex painful in any way? Are orgasms painful? Any other symptoms?
I found this, but I'm not sure if it's what you describe. Dr. Ruth wasn't much help, but it may give you a starting place if it's what you feel - https://www.chicagotribune.com/news/ct-xpm-1997-02-03-9702040186-story.html
This might be the least satisfying article I've ever read - it explains it, and offers no suggestions at all - https://helloflo.com/yes-women-can-experience-painful-sexual-arousal/
If you think that applies, you can research clitoral phimosis, which seems to have different causes, and has an equally unsatisfying amount of info out there. Definitely talk to your gyno about it, though.