You have done an increase somewhere.
Most likely a yo, to you have any holes?
but it can also be from forgetting to take the stitch off the left hand needle and knitting into it twice. Usually with this one it's tight to knit the second time.
You can decrease if you want it back to 20, just do 6 decreases...
My prefered decrease is k2tog... here's Amy's videos for all of them...http://www.knittinghelp.com/videos/decreases