LO crying n waking up from.sleep

It's 12 midnight where I am, and my 8.5 month old son who usually sleeps very well through the night, is waking up crying every few minutes. I don't know why. I havd to carry him n walk around with him n he falls back asleep. He is a good sleeper, and I can't figure out what's bothering him. In the past 4 hours that he has slept, he has woken up 3 times, but in soothing he falls right back to sleep. He is not gassy, nor feverish, nor teething. Any advice ladies?