Oh mama, that must have been terrifying.
A very similar thing happened with my son when he was about that age. I woke up to him struggling a bit with the very heavy blanket completely over his face. At first I felt like the worst mother ever, and then I realized that my mama instincts had kicked in immediately and I woke up completely within seconds after realizing what was going on. You are a very good, attentive mama for coming out of a dead sleep to help him. You are in tune with your baby, and you will not let any harm come to him, that is why you woke up when you did.
That being said, there are so many things you can do so that you and he both get the sleep that you need. My son was born in the very early spring, so it was still pretty cold. I would put him beside me to nurse and I would pull the blankets up to his waist, which was also at about my waist. I would then pile baby blankets on my exposed shoulder so that I would stay relatively warm, but he didn't have them anywhere near his face. I also cut down to two pillows on my bed. One was under my head, and one was tucked behind my back to help with nursing. I always knew where those two pillows were, because I can't sleep without one under my head, and the other one was always on the opposite side of my body from where he was. Bundle him up really well with warm jammies and a sleep sack, and he will be just fine. You can even completely uncover him and tuck the blanket down over your waist and under him to give you some extra security that he won't get them up over him again.
You are doing great, and you will be fine. Just remember, our mothering instincts are there for a reason, and you will absolutely, positively know when he needs help, especially if he is tucked in right beside you. You can do it, but be gentle with yourself.
ETA: I forgot to add one of the greatest sleep wear pieces of advice I ever got. Go out and buy a men's long sleeve T-Shirt in a size two or three sizes larger than you would wear. Cut a slit directly down the middle from the neck to your breast level and use that to sleep in. You can pop a breast out the slit in the middle and your arms and torso stay warm, and it is super easy to just fall asleep that way, or if you are still awake, just pop it back in. The fabric won't unravel or run, even when you run it through the wash, and the t-shirts are cheap enough that you won't feel that badly about cutting one, or having multiple ones if you are still leaking milk at night.