I've missed 2 periods (this and last month) and have taken 2 negative preg. tests.?

My periods have been like clockwork since I started the pill in July of '06. I do have a doctor's appt next week, but I'm going crazy. Are there any other reasons why I'd miss 2 consecutive periods while on the pill? My doctor advised me not to start the new pack this week just in case I am pregnant and it still hasn't come yet. I am married and we are the only partners we've had. Thanks for taking the time to read this!!!!