My mechanic doesn't know why my car went into limp mode.

I have a 2002 2.5L Nissan Altima (automatic transmission). I was driving ~65mph on the highway, got off an exit and waited at a red light. When the light turns green, the car only limps forward at about ~5mph. I stepped on the accelerator, but no response from the car.

I then put the car in Park, stepped on the accelerator and the engine was revving up normally. I then put the car in Reverse, it reversed normally as well. However, when I put the car in Drive, it just creeps forward, and I can't accelerate.

I then turned the car off, and waited about 5-10 minutes, then turned the car back on, everything was back to normal, the car was driving just fine. Drove for about 5 minutes with the heater on (cause it was cold), started to smell something burning (it was a liquid type of smell).

So, I decided to pull over and have it towed to my mechanic to see what is wrong with it. He took a few days to look at it and doesn't know what is wrong with it at all.

Anyone here can give me some guidance on what to do?