Sometimes with car in Drive, feels like low gear. feels like dragging. on 1995 Dodge Stratus

This start happening after I had my oil changed. I also noticed a decrease in mileage.

Might be good to have the trans scanned at a shop.
1 more answer
change spark plugs and ignition cables or coils to performance grade(E3 plugs and accel coils or cables). change air filter to high air flow design(K&N). use fuel additive(stab-ethanol treatment) to clean fuel system periodically(third tank). this may cost a lot initially but with fuel prices these days will pay off in long run.