Preflop: With 99 you are generally looking either (1) to get the pot heads up and have your 99 stand up or buy the pot or (2) see a cheap flop against a lot of opponents and flop a set.
I personally like the first option - a pot-sized raise will probably only get one caller, if that, and given your position, you will likely get the first chance to fire at the flop and take it down.
Given that you checked, you really can't continue without the set. Your chances of folding everyone on the flop are low, and even if you have the best hand on the flop, you are looking at some serious reverse implied odds
. I would have check/folded the flop.
If the flop is checked through and checked to you on the turn, then maybe take a stab at it, since no one has shown interest and even if you get called you might get to showdown without having to put more money in the pot.