Probably not a cluster headache, although it might also be a correlation causation issue, as the dark and quiet will help. The viscosity of the blood near your trigeminal nerve won't change with a hot towel over your head. The body is a decent insulator and regulator in that respect. What might be happening is that your blood pressure decreases due to the relaxation which in turn lowers pressure on the trigeminal nerve, if it is a cluster headache.
Cluster headaches feel like someone has stabbed you in the eye or side of the head and are 99% only on one side of the head, and very concentrated in one spot. The pain is excruciating and you will not be mobile at all during a cluster. Most people I've heard claim to have cluster headaches have misdiagnosed themselves, and probably just had a migraine or very bad headache. A cluster headache feels different, because it literally feels like there is something in your head. It feels as if you have been stabbed and are about to die. It ain't called the suicide headache for no reason. Personally I would only ever wish it on my worst enemies...