Very difficult for anyone to say without being there with you, there are so many factors involved. What was the temperature? how wide was your mat? Are you a cold sleeper? Had you eaten? Properly hydrated? Were you damp? Alcohol drunk? Windy? Large meal before bed?
FWIW, I'm cold sleeping at home with a wool blanket underneath me in the hammock, let alone outside.
An obvious over simplification though is that your insulation wasn't sufficient. If its lower than about 5c, I need more than one foam mat. A 3 season sleeping bag does next to nothing to stop heat conducting away from underneath you in a hammock. You need more non compressible or suspended insulation to deal with that
Cold faces can be cured with a balaclava. You'll probably find that the more nights you do in the hammock, the less noticable your cold face will be anyway.
As an example, I bivvied at under -16c this february, the first night out in a month or so at the time and my face felt uncomfortably cold. (my beard was stuck to my sleeping bag with ice)
Fast forward2 weeks to the Arctic where I spent 10 days sleeping out in the snow, it probably got down to nearly that temp on a couple of occasions and wasn't uncomfortable at all because I was then used to it, after steadily dropping temps through the week.
