I'm 19 weeks and the other day my DH and I did the business.
Afterwards I was SO sore - not inside, but i felt brusied on my 'front bum' (sorry - can't think of better way to describe it!) like i'd been kicked there with a steel toe-capped boot. It has faded now but took a day or two.
I am sort of assuming its because there's a lot of blood down there that then having an orgasm sends more down there and can make it throb....but i wondered if anyone else had had this, or if its anything to cause concern??