My 13 month old is doing the same thing, and in addition to the night terrors, my doctor also explained to me that when kids are super actively learning to use their body in all these difficult ways - like learning to walk, climb etc - during their sleep they are stirred by thinking about these movements - kind of like if you have ever gone skating after not doign it for years - how the image of your feet gliding can stay with you for hours or days.
For babies - this is learning to move their bodies, it keeps them from sleeping heavy, and can startle them to wake up - then they scream, and then realize they'd rather be sleeping- that they really aren't up and about. Like I said, my daughter does this a lot, and as a good sleeper, it concerned me a lot. But this explanation made a lot of sense to me, and now it doesn't worry me at all!! And, when I hear her yell, if I go in her room that wakes her up more. If I'm really worried I wait 5-10 minutes to make sure she is back ASLEEP before I check on her. If I go in when she is screaming - we are up for 30 minutes or more trying to calm her down. I also then try to make sure she's fallen asleep in a comfortable positions - sometimes she can be in some really odd ones that make me think she'll wake up again!!
Hope that helps!
D.