I've had some back problems (4 herniated discs each fixed by a one cortisone shot) but I've never had sciatica). I have the problem of every once in a while that I can't stand straight up without severe pain. I'm 80 and my 51 year Son has the same problem occasionally. He went to a back specialist the last time he had this problem and was referred him to a physical therapist.
It turns out that his problem (and mine) is caused by tight tendons in the hip/back. If you have this problem there are some simple stretching exercises you can do daily to keep these tendons stretched out. I can't describe them in writing so do a Google search for them.
If you've already got the pain the quick fix is to put your shoulders/head back and force yourself to stand up straight. It will hurt but after you get straight up start walking (keeping your shoulders/head back) and the pain will go away. It make take a few days for the tendons to heal completely heal so take it easy at first. Usually back problems are caused by weak back muscles that can't keep your back straight so exercise your back as much as you can.
