Hey guys, I’ve been searching around for an answer to this question but I can’t seem to find one so… My back pretty much feels as though someone took a golf club to it, and I’m fairly certain it’s from hopping around on my Uni. I ride right foot back with my right hand on the handle, which, from what I’ve read, is unusual. I just want to know if any of you can tell me whether this is something I’m just going to have to live with, or if it’s something wrong with my posture/technique that I can fix.
Riding always makes my old back feel better. Stretch, do core exercises (crunches, planks etc.) Don’t mess around with back trouble. See a specialist if it persists. At 18 you shouldn’t have back problems.
Go see a chiropractor is my only advice. It could be anything.
Is it muscular or are you feeling like a nerver is being pinched in your spine? Any pain radiating down your butt/leg? How long have you had the pain? Have you tried taking a couple/few days off from riding?
Raise the seat first, you might be bending over a lot when trying to hop. 
If not, follow other people’s advice, munivision is right, at 18 you definitely shouldnt have back problems.