Burning pain in legs only at night. can't lay on side.

See your doctor. We on Forum really cannot diagnosis someone. Be as specific as you can with the doctor about the pain, the hours you have it, after what activities, whether or not cramping is involved, and etc. This will help, along with the exam and perhaps bloodwork, toward a diagnosis. Hope you will update us when you find out what's up.

You need to get to a Doctor. There can be all kinds of reason for this.

"It could be venous reflux, also called venous insufficiency, which is when the veins in the leg aren't circulating blood back to the heart properly. In this situation, the blood is blocked up and can begin to flood the surrounding tissue, leading to the burning sensation."

Source: https://www.brownbarron.com/blog/2021/march/the-7-sensations-in-your-legs-feet-you-should-ne/#:~:text=It%20could%20be%20venous%20reflux,leading%20to%20the%20burning%20sensation.

Another source: https://my.clevelandclinic.org/health/diseases/16872-chronic-venous-insufficiency-cvi

Hope this helps!