It could be a number of issues but you should get it checked out by a professional. It could be worn out clutch discs but if I’m not mistaken, Nissan has been using a Continuously Variable Transmission or CVT in a lot of their vehicles for a number of years. While they offer smooth and sometimes peppy acceleration, when they fail they can be expensive. I don’t think it can be rebuilt so you’ll need to get it replaced. But at the very least, have an independent mechanic check it. If you take it to a dealership they’re more than likely going to tell you it needs more work to get you to buy a new car.