I've put the right transmission fluid in but the car still runs sluggish and won't shift properly. What could it be?

Back in February of this year my wife and I bought a 2012 Nissan Sentra. We've taken it in to get 3 oil changes with inspections since then. We've had no issues until just a couple nights ago. My wife was driving to go get gas on her way to a friend's house when while she was going 20mph, the tachometer suddenly went up to 4, 000 rpms. It scared her so she pulled over and turned the car off. With the help of some firefighters she was able to get the car home. I looked at it the next morning and the transmission fluid was almost empty. No lights or indicators had come on for this. Our previous oil change and inspection told us all was well. I put the correct fluid in yesterday and it did very little. The car has automatic transmission. I can get it up to about 25mph with no issues. Once I get closer to 30mph it just jumps in rpms and sounds like it's having trouble shifting. I just want to know what's going on and how much it's going to cost. This is happening when money is tight for us and at the worst possible time of year.