There are a few possibilities. One is that you are not remembering to move the yarn in back to knit and in front to purl each time, or are purling or knitting when the yarn is in the wrong place. If the yarn is in the wrong place you would be doing an inadvertent yarn over, which would increase your st count.
Is the yarn splitting? Sometimes a split st gets knit twice, so that could increase your count.
At the beginning of the row, is your yarn on the correct side? You could be doing a yo in the beginning of each round.
One suggestion could be to, of course, pay special attention to where your yarn is and for the first few inches put markers every 10 stitches or so in order to catch exactly where you might me increasing.