RPMs go up, but the mph doesn't?

I've noticed this for the past week or so at a cold start, basically after it's been sitting for a while, such as before or after work.

It starts fine, no warning lights on the dash. Shifts into reverse and drive fine. Starts driving normal until about 20mph. Then if feels like nothing is happening. The RPMs go up, but the mph doesn't. Like I'm pushing on the gas pedal but nothing is happening. It feels like it's going to stall, but doesn't. After a few moments it's back to normal and drives fine.