The bright yellow fluid usually results from an infection, and it may have drained out as a result of being upside down. In fact, I invented the technique called the Flip-Turn Sinus Flush, in which you put your head upside down on purpose, because that is the best way to get saline into all of those nooks and crannies of your sinuses, and to drain the infected mucus, which cures the sinus problem.
http://www.medhelp.org/user_journals/show/2322
Since you have had surgery, I wouldn't recommend this to you until you get checked out first. You could go to an ENT, not necessarily the one that did the surgery. The blood could just be the result of the infection, that's not uncommon, but it could be something else, so I can't say for sure.