I truly also don't know. What i would say is two things.
One ok FWB.. there is a comfortable stage there.. You let your body fully go, you orgasmed, so did he.,... then you realised your body so let go, that something else came out... He said it's ok... Don't cry... It means that your body was relaxed and let go.
As I said, hasn't happened to me but i can understand it happening.
The other thing you need to consider, is the way you have described the situation, it was minimal, ... you may be lactose intolerant, you may otherwise have a weekness there, like bladder control. Nothing to be ashamed about, life is life.. It's not life threatening hey!
So, do me a favour and go see your Doctor and mention this, so that they can ascertain if you were purely "orgamic" smile!!!!! Or, have a slight weakness that you can strengthen up.
Don't think that it is going ot happen everytime, I am using "gut" feeling... Didn't happen before... But you do need to ascertain if it may be a future problem..
Don't stress, we all have some form of problem, better this than I can't orgasm, or feel a thing hey?