That is a reasonable worry. How hot is it? How cool is the night expected to be?I was worried it would be too hot if anything. It's hot here in California and I'm scared of killing it.
It usually works best to provide a warm place and a cool place, and let it move around to find the right temperature somewhere between the two. But if you don't have a cool place, that is much harder.
It's usually best to have a heat source it can go to, and go away from. Don't try to stick it on the heat source and keep it there. Yes, from what you describe, it probably was too hot.Would a sweater work as a 'heat source'? I have this microwaveable like beanbag the size of my hand but it didn't like it much and started opening its mouth and its throat moved in and out.
Sweater, like the thing a person wears? It might like to snuggle there for comfort, but if the night actually gets too cold, that won't help much. A single baby guinea just does not make much body heat. Whether it will be enough really depends on how cold the temperature gets at night. Do you have an expected night temperature? Maybe from a weather forecast?