Auto transmission problems

My fiero has 100k on it and we just changed the transmission fluid and filter. It was driving fairly well before we changed the filter and fluid but now it is acting up. I have to get the car up to about 4000 before it will change gears. If I shift it manually it will change between 3000-3500. I wasn't sure if this would be a bad transmission, kickback cable, or maybe if I didn't get the filter on tight enough. Any help would be much appreciated.