Could it have been an insect bite/sting? Maybe something crawled under that part of the mask and got pinched. I'd expect an allergic reaction to me more itchy than a bump.
I got worried:
You woke up without your mask and a knot on your head... First thought was, were you in an alley, behind a bar, next to a dumpster, missing a kidney?
Then I read the rest.
Does that red mark match up with where the mask fits against your forehead? Can it be an allergic reaction? Skin irritation? Or does it seem more like an impact like you hit your head against something during the night while you were asleep and knocked your mask off?
I remember a time I rolled over after sleeping funny on my arm. My arm went dead and when I tried to move it, it flopped down and smacked me in the face.
Just came across this accidentally. Most likely an answer has been found, or the OP has given up looking, but what the heck.
Many years ago, before I discovered that I had sleep apnea, I also would wake up with a large red spot in the middle of my forehead. Eventually, after a matter of months, the spot turned a darker color, and didn't disappear after a few hours.
What it was, in my case, is that I would rest my forehead against the bed headboard, with neck angled slightly upward. Apparently this helped me breath, as eventually I would unconsciously do this every night.
After the apnea was diagnosed, I quit propping my head against the headboard, and the now dark perma-mark slowly disappeared.
Your problem may be entirely differant, but at least it is something for consideration. Maybe your CPAP theapy is not working 100%, and you also get some breathing relief by propping your head against the headboard, or maybe wall.