problems with casting on additional stitches

[hennrietta]

So the part of the pattern I'm working on goes like this:
Row 28: purl all the way across. At the end of the row, cast on 5 stitches.
Row 29: knit all the way across. At the end of the row, cast on 5 stitches.
Row 30: purl

Well I did row 28 and 29 fine, but when I try to purl the new stitches, I get a long string of yarn between the needles. I didn't cast on anything fancy, just made a loop and slipped it on the needle (I'm a beginner at this whole knitting thing, and don't know all the proper terms yet). I don't know what's wrong, but I didn't have this problem when knitting the first 5 additional stitche. Any ideas?