Medically, what causes the pain after sex?

[OneBluntZebra]

I am menopausal and have endometriosis. Sometimes when I have sex my lower back is in excruciating pain the next day. This starts in the morning after and this last time it has taken a full week for it to get better. The pain starts in my lower back and moves around, eventually getting to my lower stomach area and then stopping.

Although it would be lovely to think this is due to my athletic bedroom activities, sadly it is clearly something else and I am blaming my endometriosis. How, from a medical/scientific point of view, could a combination of endometriosis and sex cause this?

Endometriosis causes adhesions, scar tissue and generally makes tissues and organs that would otherwise be free to move with a bit of pressure from sex.
With sex, these tissues can't move so instead get pulled and prodded, and the cramping with orgasm (even if we don't feel it, our cervix contracts) can make it worse.

Another thing we forget about are the nerves. Our pelvises have so many nerves clustered in a small area. Any slight swelling can really impact them.
I had excruciating pain with sex. Every thrust felt like sciatica in my vagina. Doctor was convinced it was endometriosis. It was a trapped pudendal nerve.

ToBeOrNotToBee Why would that make my back in so much pain for so long?

You're also getting older. The gaps between the discs narrow with age and the musculature/core strength is reduced with lower levels of physical activity and lack of flexibility training. Your activities could be both causing compression/impact of the spine to squeeze on nerves, along with stretching and flexing exercises causing muscle pain.