2002 Dodge Dakota sudden gas milage decrease....HELP!!!

I recently had my check engine light come on. a week later, I got an oil change, air /oil filter, and fuel injection. they said the CH. eng. light was on because my gas cap wasnt closed all the way and reset it. They said too, that the fuel inj. would improve gas milage. About a day later, the light came on again, although my cap was turned a few times. as the week went by, I was realizing that I was going through gas like crazy! I went back and they said the end of a hose to the o2 sensor was cracked and fixed it for free, then reset the light. Well..again a day later, the check lights on and Im going through gas like MAD! My gas milage was so great before--Whats going on? PLEASE HELP me with some idea of what's REALLY wrong before I go back there. Thanks