1994 Oldsmobile Cutlass cutles is shifting in to lower gear

when i got the car about 6 mo ago i was toled it sometimes when it was cold would do what they called nutral slaming it self never herd of that latly it has been do this a lot is what it does is when driving it will shift it self to what seems to be a lower gear when it does this that spadomiter goes nuts and the rpm go really high around #4 the if i stop and try to go its like its in the lowes gear posibal and is hard to get it to go but gos really slow we just did a trans fluid and filter change and when it shifted to this lower gear iyou felt it its not as hard now but still doing it what couled it be