They probably asked to see your regular doctor because there are soooo many things to throw your cycle out of whack. Stress. Weight gain (or loss). Hormones. Illness. Not everything has to be related to the "female parts" to make you late. Its just as likely to be something completely different, but affects your cycle.
Glad the test came back negative. Its ok to miss a month - its not ok to ignore it. Find a doctor ASAP to figure out why you missed. I always had irregular cycles but always knew why, so when months went by and I didn't have a period, I didn't worry. Like Jenn, I enjoyed being free! :D :p