3 year old weeping for no reason

He puts face in between his arms and lays down face down, and weeps, and cries, after he's done (usually 2-5 minutes) he either sleeps, goes back to playing> in a bad mood, or clings to me and I comfort him, then he goes to sleep.
I dont know why he does this, we love him to death, provide him with everything he needs, he has a little brother, 1 year younger, they play together. We always play with him, teach him, and talk to him. We reward him when he does something right or accomplishes something.
He is an extremely bright and intelligent child.

I just dont understand why he's been doing this.

I've seen him do this three times now.

[Kevin Kennedy, Ph.D.]

There is a reason for the behavior - it may not be apparent, though. Be sure to have him checked out medically to be sure nothing is being overlooked.