It sounds like one of two things might have happened. Or both!
When switching from knit to purl, you must bring the yarn forward between the needles (not over one). To switch from purl to knit, you must bring the yarn back between the needles (not over). Did you forget? If you did, you made 'yarn overs' that can sometimes look like they are overlapping other stitches on the needle.
The other possibiilty is that you twisted your cast-on row as you worked it.