Problems with my 1999 S-10

I have a 1999 Chevy S-10 and I've been having problems with it, and I need a little advice on what may be wrong with it. It has 94,000 miles, I just replaced the plugs, oil and filter, and alternator. I have been having problems with the car backfiring, seems to be missing when its idling, and especially when I accelerate. Although the timing is non-adjustable, from my knowledge, but it seens to be off, or mis firing on a cylinder or something. I don't know what else to say or do. Any advice would be great. Also, another thing I forgot to mention, all of a suddon about 3 or 4 weeks ago my gas mileage went from 20 mpg, to 8 or 9 mpg. Big problem for me, any suggestions please.