Scary!!! That would have freaked me out too.
There are a few things I can think of here....
You and DH were "really tired."
The baby was on a pillow.
The baby was between you and DH.
Cosleeping is not normal for you, baby or DH.
All of these things are more dangerous because cosleeping isn't something you normally do, so you're not hyper aware of the baby like parents who cosleep regularly are. That's one reason why I think that baby between you and DH was risky - he's not used to this either.
Not to mention you both were "really tired".
Plus, never put baby on a pillow.
That all said, I feel badly for you. Scares like that really unnerve a person - I'm glad he's okay.
I'm a very light sleeper. Henry sleeps in the crook of my arm swaddled up. When DH is home he sleeps between me and the bed rail. I sometimes put him in a sleep wedge.