needing help diagnosing

tonight i was filling up at a gas station and when i got back in i turned the key to start it and there was a loud knock which im assuming was a backfire but im not postive... but after that i turned the truck completely off and tried starting again and all it would do is turn over. i waited about 2 mins or so and tried again and it crank up but it took longer to do so and once it was started i shook real bad. i drove it home and when i got home it smoothed itself out.

does anyone know what this could be? im thinking maybe a fuel issue due to when i filled up i calculated my gas mileage and it had dropped considerably from 16-18 to 13mpg.

the truck is a 1996 s10 with a 4.3 vortec v6 (x-code) with about 131,xxx miles

Have you checked your fuel pressure ?
Put a new fuel filter on it, $10 & cant hurt it.