I would have to mostly agree with caskConditionedAle above. I doubt very much the seat alone would be the cause
of your back pain. But you could try a few different seat settings and a lumbar roll to help relieve it. Once your back settles down (back pain can often last 6 weeks if it is muscular damage) then I would start doing some exercises/yoga recommended by your physiotherapist to strengthen and make more flexible that affected area.
About 3 years ago I started having what I thought was sciatica pain down my right leg whenever I was driving - and it mattered not what vehicle I was driving. Removing my wallet from that side rear pocket greatly helped. I then read this:
and since doing some yoga style stretching of that piriformis muscle I have almost completely relieved the problem.
I hope you find some relief whatever you do...