Most likely you're making inadvertent yarn-overs. If you knit a stitch with the yarn in front, or purl a stitch with the yarn in back, you'll get an extra stitch.
Make sure that if you knit, the yarn is in the back, and when you purl you bring it between the needles to the front.
If you're using a funky-furry yarn, the strands can be deceiving, as well, and you knit into the fur that wraps around the needle as well as the thread that holds it.
Most likely, it's one of the two above things, considering how many stitches you've increased in such a short time.
Hang in there! It gets easier!
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry