What you're experiencing sounds perfectly normal to me, and unless we both have the same swamp fungus growing in our bottoms, I wouldn't be too worried. The lining of your rectum and colon is fragile and continually being regenerated. When douching, and with deep anal play after, this lining can become dislodged and ejected and appear to be mucus-like. I find it is very common if you have sex too soon after douching, before your body is able to absorb the excess fluid that remains.
It can act as a "natural" lubricant, which is why I sometimes will douche deeply hours beforehand, then again very shallow within 15 minutes or so - plain water only. It results in the experience WatchStr8Sex described.
I'm going to counter what's been advised by others here. Unless you have excess discharge without douching, or there is something other than a clear-whitish discharge, I would not go out of my way to see a doctor. A mention of it at your next regular visit couldn't hurt, especially at my age when they're doing a prostate fondle anyway.