I think Julie is correct. Can you link to your pattern? Think of the circular needles and "needles on a leash" that keep the one you drop easily retrievable.
Knitting Flat With Circular Needles
Watch for something like "join to knit in the round" on patterns to know that it's supposed to be knit in the round. You can also ask here if you aren't sure. There are definite advantages to using circs for knitting flat. I rarely use straight needles or double pointed needles, interchangeable circs can handle it all.