Sorry to hear that @Cream Soda.
Standing up for long periods gave me a whole load of trouble in the mud bath that was 2016. Very few spaces to sit....and I'd lost my shooting stick, which I'd used to help rehabilitate a broken ankle back in 2011 and taken to Glasto ever since. Back got so bad in 2016 that one night I nearly fell over, as my back kind of gave way. Problems persisted after I got back home. I eventually had an MRI scan which showed damage to a disc in my lumbar spine. Whether I had a preexisting injury that was exacerbated by Glasto, or whether damage occurred during Glasto, I'll never know.
I really sympathise with you, as back pain can make a festival a rather miserable affair at times. For next time I'd really recommend you invest in a shooting stick. Fairly lightweight and you can rest your back wherever you are.
More to the point, if you were experiencing such extreme pain I would be tempted go to your GP and push for an MRI scan to check for damage. Pretty important to check for this and rule it out.