So I'm just starting out and have been riding my (fairly crappy and too small) 26" hardtail (pic here) on trails/singletracks the last couple weeks, usually about 10km. Due to somewhat increasing confidence and fitness last Saturday I did 2 x 12km in the morning and afternoon, riding a bit faster than normal too. The trails are a bit rocky but mosty very hard-packed earth with some roots, nothing particularly hardcore at all.
The last two days I've been laid flat out in bed taking painkillers with bad lower back pain. It's not muscles but feels more like my spine, maybe vertebrae got bruised or out of alignment or something. I'm tall (6'3") and have had on/off back problems in the past.
This bike was only ever a temporary one, and I'd been planning to buy a new steel 29er hardtail (something like a Surly Ogre plus front shocks, or On-One Inbred sort of thing) but am now rethinking this. Should I get a FS instead? I really did not want FS (expense, look, maintenance) but if riding is going to mess up my back like this maybe I should reconsider. Would something like a Thudbuster help do you think?
Or is it just bad technique? I've been standing up on the bumpy downhills but I find it hard to stand up on the bumpy uphills and prefer to be seated.