backfiring

[blind34_1]

yep my car will backfire on average twice in a fifteen-twenty minute trip. I just recently had timing belt changed, and fuel filter. After I got the timing belt changed, my car felt like its slower, like it takes more gas to go. It doesn't have the get-up it used to have. Does this mean that maybe a belt is too tight? or is it just the newness of the timing belt? could the filter be suspect?
maybe injectors? I don't know. I want to come back to my mechanic and tell them whats wrong.