Why is my 8 month old suddenly waking at night?

I have an 8 month old baby, and for about the past 3 weeks he has been waking 1 time a night, anytime between 2 and 4 a.m. When he wakes up, I give him his pacifier (while keeping all of the lights off and not speaking to him), and he will go back to sleep. Sometimes he falls asleep immediately, sometimes it takes up to an hour. We just took him to the dr. yesterday and she said that he is in perfect health and that she doesn't see any signs of teeth coming in. He has 2 teeth already, but I don't believe that he has any more coming right now either..I thought teething pain may have been the cause but now I don't have a clue as to why he is doing this. He has been sleeping through the night (8 p.m.-7 a.m.) since he was about 2 1/2 months old. Is this just a phase he is going through? Am I rewarding him by giving him the pacifier? What can I do to help phase out this mid-night waking? He sleeps in a crib, and has since about 2 months, and his room is always kept around 70 degrees, he's warm enough but not too hot, and we have a humidifier and fan running (not pointed towards him!) to drown out noise. He also sleeps with his stufffed elephant, which he loves very much ;) Thanks!!