Make sure your shorts really do fit. They do wear out (in a year or two) and weight gain can make them fit differently. Plus sizes can vary between manufacturers, even year to year some times.
Make sure you saddle fits. They wear out over time too. Try some different saddles, most places have demo saddles. The Terry Butterfly is the one I know that most women swear by.
Get some chamois creme. I've always been a fan of the non-vasoline based ones. Sportsbalm and DZ Nuts are my favorite (they make a women's version). The Assos stuff is nice too but $$. I put mine on the chamois and not myself, if that feels less weird.
Get out of your shorts ASAP after a ride and get cleaned off. A lot of saddle sores start that way.
You may need to have your current saddle sore addressed. If it's just an ingrown hair you can often let that heal by itself or address it yourself. If you really have a saddle sort (infected sweat gland) they can get bad very quickly and require a doctor's care. In any case I'd make sure the current problem is under control and then start eliminating the other variables.
You may also find this place helpful:
http://forum.slowtwitch.com/gforum.cgi?forum=7;
There are plenty of threads addressing all sorts of female specific issues with plenty of good advice.
Good luck!