Why am I experiencing Low gas mileage after changing my spark plugs??

What is your question?

I have very little experience in working with cars to this point but i decided to give changing my spark plugs a shot. The front 3 plugs were relatively easy to change, but in order to be granted access to the back 3 i had to remove the upper intake manifold. After doing that I switched out the plugs and put it all back together. The car seemed to be running fine last night, and this morning for that matter. I noticed that my Mileage Per gallon reading on my car is significantly less than it was prior to doing my work on the car. I used to get about 21 MPG according to the car reading and now I am currently getting about 11-12 MPG. Any ideas??? No check engine lights on or anything???