Can you take a picture of the hen's head and post it? That would be the quickest way for us to give you an answer.
If not, frost bite is going to start at the tip, the point smallest and furthest from the head. If the spot is on the side of the comb, surrounded by healthy, red flesh it's likely a scab. A scab is also going to have a sort of raised / on-top-of-the-flesh appearance, at least somewhat, and also tends to be very dark whereas frostbite will be dull and look to be "one with" the flesh around it.
Now, that said, turn the heat lamp off and keep it off. You're not doing your hens any favors and -- whether what you're seeing now is frostbite or not -- you are absolutely, positively increasing the risk of your flock getting it now or in the future by using the lamp. Warmth put off by a heat lamp does not keep moisture away, it encourages it. And with the on and off pattern you're inhibiting your flock's ability to acclimate to the cold on top of it. They're far better off left to their own devices. Really, they will be just fine.