It sounds like while you were cold underneath, your muscles were contracting to generate heat and you couldn't relax. No wonder you woke up sore.

If the pad you're on is comfortable when it's warmer, then it sounds like you need more insulation for when it's colder. More insulation underneath doesn't cause overheating nearly as much as insulation on top, so don't worry about a new pad or pad combo being too warm. Venting or using less insulation on top is a much better problem to have.

A foam pad may be a good intermediate step to prove the concept before you sink money into a warmer pad. Also, they're good multi-use gear: sit-pad, protection for the air pad, staging area for gear...