Hi Kevmos27, the good news is that this sounds very normal and you are one of the 85% + of people who experience this at one time or another. The fact it is mid back in sitting and is relieved when you stand suggested you have probably a mechanical issue in that region, probably the result of too much sitting slumped and therefore two much repetitive load on that section of the spine.

The remedy if this is the case is going to be to try and restore your spines mobility from top to tail, both its ability to flex (bend forward) and extend backwards, side bend with rotation both left and right. It's not difficult to gain some more mobility but it does take a little time and progressive exercises/stretches to get there. Manipulation is effective would only be very short term. Anti-inflammatories would be of little to no use as you are probably not dealing with inflammation. It is mechanical and so a simple pain killer e.g. panadol might give temporary relief but certainly no cure.

While you are waiting I would invest in some deep tissue massage to the spine, maybe two sessions a week for 2-3 weeks. A studio based pilates class (using equipment to help mobilise the spine) would also be a benefit. Basically you probably just need to invest some time and effort in unlocking what has being to get a little stiff so you can stop aggravating the segment that might have taken a little to much load and might have become a little too mobile (hence the spasm to try and block the annoying movement).

Don't get depressed about it, it is very normal and I have been through this type of thing myself from time to time. It's a wakeup call to keep moving, the swimming is great but then depends how you swim. Some backstroke to help balance things out as you get more mobile could really help too.

Hope this gives you some assistance. As you know this is a forum, so we can only offer an opinion and perhaps a course of action based on words and not visual and physical assessment. But from what you've mentioned I do agree with your GP that MRI/X-Rays etc would not be indicated at this time. You could invest in a foam roller and lay lengthways on it for 15-20mins daily as a way to give the spine a rest. There's a load of exercises your Physio can then give you once you see them to use with the roller.

Let us know how you get on in due course.