Up all night worrying.....

My 3 y/o son has been crying in his sleep. We will run too see what's wrong and he's fast asleep. It scares me so bad. It makes me feel like we're not being good parents and are doing something to cause it. It makes me feel guilty, thinking that getting a job and not being home is too stressful for him and is causing it. I tried to look it up online. I found something on webmd that said it could be night terrors. I don't know though. He doesn't sit up or thrash around. He just cries out. I can't stop thinking that whatever it is it, it's my fault.