Are you sure it’s not just lathered up lube? It tends gets all worked up and looks like discharge, but it’s just the lube coming out as a blob.
If it’s not that or semen, or semen mixed with lube, then there’s a possibility it could be a type of mucous that the body creates naturally in response to constipation and sometimes anal sex triggers that secretion. If that’s the case, there’s still no cause for concern. It’s normal, not unhealthy, and won’t hurt you or her.
You should tell her what happened anyway because if it continues, then there might be something going on with her health-wise (IBS or something) and she would want to get it checked on. No need to freak her out, but you have to be up front about it!
I’ve heard about it happening a lot so I don’t think there’s something wrong, but I’m not a doctor so this is just one layperson’s opinion.