2 back surgeries in ~1 month. wondering if increased swelling@l5-s1 from overdoing it could cause severe burning/cramping pain behind knee, in calf/ankle, & thigh? have to lie down & hard to walk/sit.

Yes: Overdoing so soon after back surgery can cause nerve irritation which will result in symptoms the you descibed. You have to take frequent break and lie down to get relief. Good Luck

This may be related : to anyone of several possible causes. Sometimes, even though the nerve that may have had some compression on it does not immediately respond to surgery or if you had surgery especially 2 so close that just manipulation of nerve may cause inflammation . Also one has to make sure you did not develop a blood clot in leg post op. You should discuss this with your surgeon. Scar tissue can form now.