2002 Nissan Altima sudden drop in speed

my car's speed drops suddenly while I am driving.it seems that it slips out of gear.I press gas but it feels there is no respond to that at all.to check for the gas flow I put it in nutural and pressed gas, it was fine and engine was taking the gas. I put it in gear and start driving, it was going may be 2 miles per hour.It did not matter how much I pressed gas still going with that steady speed it was like gas peddal was not working. I had an engin oil change eairlier that day. this started to happen off and on.What seem to be the problem? Is it possible that tranmission oil pan was drained misstakenly instead of engin oil?