I agree with Aussie and Midstudent, that maybe you are retaining a little bit of water up there which is then being released once you're relaxed and turned on - along with a little bit of the brown stuff because the water has helped it soften slightly.
I know this is contrary to the fashion (or good practice, however you see it) of douching but I actually prefer not to douche, and have had no problems with messiness when receiving anal sex whatsoever. Nothing is kept in the rectum, it's just a passageway. Poop is kept in the bowel, which is way further up and can't be made contact with during anal sex. If you are a normal person with a healthy digestive system then there is no reason to have to douche, unless you want to. Like me you might find you have a nicer, completely clean experience without douching, and the associated residual water and runniness in your system.
You mentioned you douched a good half hour before the meet. I don't douche as I've said, but my understanding was that anal douching needed to be done 2 hours or so before bum fun? If you definitely want to douche maybe you need to do it more in advance to avoid the side effects?
I actually ask men not to douche (but instead to go for a number two first, then wash thoroughly) to avoid the risk of runny brown water coming out when I am using my strap on on them, which has happened before. I personally prefer a nice normal undouched bum, as do a fair number of others. Remember also the mainstream fashion for anal douching started up very recently... internal cleaning is a trend in which you don't have to participate if you don't want to. xx