1998 Pontiac Firebird transmission shifts late

Hello, I have a 1998 Pontiac trans am with a 5.7 and 80,000 miles with an automatic trans. Yesterday as I was leaving work, merging onto the interstate my car did not want to shift. I pushed on the gas and the car got to about 45 mph and would not go any faster but the rpm was between 4000 and 4500 and the check engine light came on. My first reaction was to let off the gas and when I did the rpm immediatly fell to its normal for that speed range (approx. 1500 rpm) and I felt the transmission shift (not hard, but noticable). I pressed the gas and the car got up to speed. I continued to play with it on the way home and what I am seeing now is the car continues to shift, but it is doing it at higher rpms than normal at all speed ranges with normal driving. If I really get into the gas pedal it is even worse. But as before, when it starts to rev higher than I think it should all I have to do is release the gas pedal and it shifts and the rpm goes to normal. It is also shifting harder than it did before this happened. Again it is not an abrupt shift, it feels like the car is having to almost work up to shifting and when it finally does you can feel the car surge forward a little. The last thing is, it does not have nearly the pep when I go to pass someone. Before this at 65mph I could hit the gas to pass someone on the interstate and it would still push me against the seat, now I think my wife's 3.0 ranger has more pep at high speeds. I know enough to get by with cars but admit to knowing absolutly nothing about transmissions other than checking the fluid and changing the filter. I really dont want to get ripped off at a tranny shop, especially if it turns out to be a 30 dollar sensor. Any and all help or advice you can give would be appreciated. Thanks