Ah, sleep issues. This is a problem that is near and dear to my heart. Yes, for some reason at about that age, it seems like sleeping troubles begin to creep in. I experienced this with both my boys at around 16-17 months, but the way I dealt with it has made a world of difference the second time around. Read Tammi F's response. She hit the nail on the head.
With baby #1, he was always a good sleeper until about 17 months when he began waking around midnight every night. I was worried that something was wrong. I would get up with him, hold him, walk around with him, sing to him and whatever I could possibly do to comfort him back to sleep. I was 8 months pregnant at the time. This went on for MONTHS (when baby #2 came, I was dealing with them both all night long and hardly sleeping). Even to this day, at three years old, he winds up on the floor in my room most nights, and refuses to sleep in a bed (only on the floor). His sleep has gotten better, but I feel like if I would have nipped it in the bud we would not have run into such problems with his sleep habits (sleep training is sooo much harder at this age).
So, here comes baby #2 who was a terrible sleeper early on. We had a new pediatrician who suggested sleep training for him at 6 months old, which worked wonders in getting him to sleep like a champ. But, same thing...at 16 months old, he woke up during the night for the first time ever since he began sleeping through the night. But this time, I remembered back to my sleep training days with him and followed the same routine (Just the way Tammi F. said to do it) and after that day, he woke screaming again one more night, but that was it, and then it was back to his normal 12 hour nights.
Actual night terrors are a pretty rare thing. They are related to sleep walking, and if your son is having them, he is actually not awake when he's screaming. I think separation anxiety or a bad dream are more likely explanations, but he does need to learn to put himself back to sleep or you will actually be feeding into a sleep problem (maybe even for years to come). If you think teething pain is a problem, give him a dose of Motrin before bed, for comfort. Even if teething is the cause, it is important for him to learn to sleep though it.