I've had this happen before, both 'naturally' and as a result of my attempts at lucid dreaming.

While attempting to attain lucidity, my dreams would often "fool" me, by having me dream that I was waking up. I would then realize I was dreaming again, only to dream that I was waking up again. I have had this happen up to 5 times in a dream... when I finally woke up, I was highly disoriented for most of the rest of the day.

From my experience, events that would tend to give you 'deja vu' (i.e. things that you perform often, or are repetitive in nature) are most likely to create the feeling that you are dreaming, when you are in fact not. This happened to me the most when I was working long hours at a job that bored me to no end...

The way I figure it, things that you perform over and over (lecturing in your case) become ingrained in our thoughts. In the same way that muscle memory develops from working out, "mental muscle" develops familiar channels that our thoughts tend towards. This can heavily influence dreaming -- for instance, it can be common to dream about working. Also, if you spend time thinking about what you are going to be doing the next day before you fall asleep (i.e. lecturing) this can influence your dreams.

After waking, you may not remember the dreams, but the memories exist on some level. When you perform the familiar action, it triggers memory recall, and you will have a flashback of varying intensity. Knowing that the memory is false makes it confusing to your brain... hence the feeling that you are currently dreaming.

Ways that I have found to combat this, include strengthening your dream recall ability, and adding variety in your waking life. Find the most repetitive things in your daily life, and change them up. Besides helping this issue, it will probably add a new level of interest to your day!

However, I must note that I have always been able to tell that I am in fact awake - at worst there are moments of extreme confusion, but they have always passed within a few minutes. If your symptoms are worse, examine your personal life. There may be larger issues at work here, although I personally feel that schizophrenia may be a little too extreme of a diagnosis.

Hope that helps.

-panzer