Try slow down or shift up
 

I have a 2004 Volvo S60 2.4 When coming from school one day I felt a pull on my tire and I thought it was going flat then I heard a knocking I know I have to get an alignment but I thought the tire was going to fall off then a message saying "try slow down or shift up" and the check engine light came on and when I got out my dad said it sounded like the muffler was knocking . The next day on my way to drop my dad off and for me to go to work I was driving and I stepped on the gas peddle and it went down like it's supposed to but I didn't go faster I slowed down and the car jerked back and I pulled over and it wouldn't let me go above twenty any suggestions or clues why this happened

do you have a check engine light or warning now? If so, you need to read the OBD2 codes to get some clues. If you don't have a scanner, check to see if your local Autozone will read them. If you bring your VIN they will also match to possible parts for your car (which is why they read codes for free)

Stop driving it and take it to a mechanic.

There is probably a mechanical failure inside the transmission.

update 1
 

I've taken my car to the mechanic and it was misfiring and needed some parts

Good to hear the problem is fixed.