if the pain is in the centre of your spine... Dont mess with it. Seek professional help. Dont go to your Gp they aint got a clue. Privately see a chyropractor if its posture/skeleton related or a physio if its muscular. In the mean time over the weekend get hold of a tens machine from boots. Place the pads on the affected area and turn up as much as you can stand. It will trick the muscles into relaxing from the spasm, if thats what it is.
I would def say seek private help, do you have any idea of the underlying cause? For instance, degenerative disc disorder, very common. Compressed vertibrae, curved spine(shoulders left or right of hips) slouch(poor posture)one leg longer than the other(most people do, the body compensates if its within 10 mill, more than that can cause problems) all will cause the soft tissue to compensate for stratural problems, this compensation usually involves spasm as part of the compensation process. In other words, is a sign of more serious problem that can usually be sorted easily if treated early enough, or if an injury then you probably will have to manage it yourself as required as i do. But if youve just been gardening/bend over alot, then simple lifting bending techniques can prevent further problems.
Hth