Maybe she was on a scavenger hunt?

You been finger raped dude!

I had this happen a few weeks ago.  I the midst of an oily HJ in it went but, dang, it just about sent me over the top, really liked the spontaneity of it all.  Different vibes for different tribes.

I think plunging a digit into ones anus needs a heads up unless of course she plainly states digital anal interaction encouraged and accepted.  

I'll bet she laughed her ass off when you left.

up my ass if she likes :p    as long as she really knows how....

new things can be part of the fun, here. Of course, you DO need to open your mind before you open your sphincter.