I can't endorse stitch markers highly/strongly enough; they've saved my sanity too often! Use them for marking edges (borders) or each repeat. When you get to rows with more than 200 stitches (for example), even if those rows are nothing but K200, PM every 25 sts. That way, when--not if--you're interrupted during the row, you won't have to count all the way from the beginning of the row to know where you are. Put one at the center of a row if, for some reason, knowing where the center is is important.
Stitch markers are an incredible investment. For very little money, they yield an amazing return in confidence, knowledge, and skill development. I only learned to knit in May 2011 but learned about stitch markers almost immediately. I was knitting lace within four or five months b/c I knew I wouldn't get lost.
I'm sure you'll figure out the source of the extra stitch very quickly with stitch markers!