Backfire?!?!

Alright, I was driving home from my friends house and in order to make this yellow light I hit my gas pedal, not hard but half way and instead of an imediate acceleration like it normally does I here a big pop(during the pop no acceleration) and then I go. The pop sound came from my air intake which I had installed about 4-5 weeks ago. This is the second time this has happened to me in the last 2 weeks. Do you know what could be causing this? What could it be? Any ideas would be great.
Thanks Mike