It's common for new knitters to pick up extra stitches. What usually happens is your left needle is accidentally picking up the stitch between or you are doing an accidental YO (yarn over). Both methods are increases often used in patterns for shaping or decorative effect.
I suspect you are adding accidental YO when switching from purling to knitting. If you leave the yarn in front after a purl and then knit you've got a YO.
So simple answer is be careful where your yarn when switching stitches and watch where your left needle is so you don't pick up the stitch between.