Blood in mouth when I wake up 7 days post-op

I'm 7 days post-op, and things are going pretty well. Minimal pain and swelling is gone. One thing that's concerning me, though, is that every morning since surgery, I have woken up with a brown coating of what I assume is blood on the roof of my mouth and tongue. It has gotten better since it started, but it is very confusing because I have no bleeding during the day. Any ideas as to what could be going on?