Talk with your doc... but 99% it's a nerve getting squashed (the other possibility is swelling / edema that can be taken care of... but that is a sign of toxemia/ low blood pressure, etc.). They'll know right away because they take your BP and do a urine sample every time you walk in. But 99% likely it's just a pesky nerve.
DO try paying attention to how you sit. You've got weight and "stiffness" in your middle... so we all start sitting differently. It may be as simple as you squishing the nerve yourself. (I do that all the time... I'm in chronic pain so when something flares up I don't even notice I've adjusted the way I sit/lean, until I go to stand up and realize I've leaned on a nerve and am tingly or numb/fall down).