What causes a 2002 Chevrolet Cavalier to use a lot of gas when it did not before?

change the air filter, and spark plugs and if the oil hasn;t been chaged latly

i have a 2000 2.4 it does the same thing its either the plugs filter or the o2 sensor take car and get a obd2 test and see if a catalyst code comes up and that's your issue.

if you have platnium tipped sparkplugs before you change them have them pulled out and see if they just need to be cleaned usually has to be done every 45k