Blanket help!

[apeterson711]

http://www.lionbrand.com/patterns/50073AD.html

I'm trying to make this blanket - since it says beginner, I thought it'd be easy enough but I'm having some problems! Once I get to row 18 is when things go weird. I always end up with extra stitches. Perhaps I'm doing something wrong when I switch from purling to knitting and vice versa? or I'm picking them up wrong? I would greatly appreciate any help, before I go crazy