Well, if you want to know what happens to the body immediately after death, you can google that. There are articles that discuss the process of decomposition in a corpse too and they tell you when rigor mortis sets in, why, and how long it takes for the body to relax again (the body does not remain in rigor mortis).
I haven't figured out how to add links on to posts yet, but on the website How Stuff Works: Health, they have an article about rigor mortis.
It even mentions that if a person is engaged in a strenuous activity when they die, rigor mortis can set in immediately (so, in your fantasy you described, if you are struggling or even going along with it, when you die your vaginal muscles may remain tight as rigor mortis sets in immediately. And don't forget, depending on the type of death your muscles can spasm as you die adding a bit of fun).

I'm lucky enough to have a friend who is a first responder and this person tells us stories and loves to share their know-how, so I've learned a bit from them.

There used to be this show on TV...(I think it was Dr.G. Medical Examiner)...it was about a medical examiner. It was one of those documentary things on one of the learning channels, so it wasn't a scripted show at all. It's more like those ER shows where they tell stories from the ER, only it was her job to figure out how people died by the clues left on the dead body.
It was fascinating.
You can learn a lot from shows like this. There was also this other show...
I just found the website for it! Awesome! (Now I know what I'll be doing on my free time....)
1000 ways to die on Spike.com. I'm sure you can google it too.

It goes into morbid, random, and Darwin-awards deaths. How they died, why they died, and what happened to them when they died.
I don't know how much detail they go into in terms of...you know...the fun sexy parts. But it could flesh out your fantasies as you learn more about what happens to the body during death.
Of course, in our fantasies and imaginations, anything can happen! So don't let reality ruin too much for you ;)

Happy learning! :D