Understanding the Infield Fly

[Mass Ump]

I umpired a Fall Ball game last Saturday and there was a quick pop-up that probably went no more than fifteen feet up in the air and to the left of the pitchers mound. There was one out and there were runners on first and second with one out. The pitcher bobbled the ball and then the bases were loaded. The coach of the defensive team came out of the dugout pissed off that I didn't call the infield fly. He actually yelled why isn't that an infield fly in a demanding way. If he had called time and then asked me why I didn't call it I would have gotten both coaches together and then maybe would have called it. I asked the offensive coach between innings how far he thought it went in the air and he said no more than twelve feet. Its the first time that I have been called out on not calling and infield fly since I started umping two years ago. Generally on an infield fly the ball would go alot higher in the air that way you have plenty of time to call it and you see all of your surroundings better to do with runners on base even though I know your suppose to know that before the pitch. Do you think sometimes on such a quick pop-up that doesn't go that high that it always has to be an infield fly.
It all happend so fast I really didn't have much reaction time, I guess live and learn on this one. Could anyone give me some input on this one.