Why have I shrunk in height?

Everyone shrinks as they age . You're a little young yet to have shrunk much but it is absolutely normal to shrink as you age. I'm 55 years old when I was about your age I was 5' 7", but today I stand about 5'6". And it doesn't have anything to do with you lifting heavy boxes. It has more to do with your spine compressing in the natural state, doesn't mean you have something wrong with your back .

I am the same, I used to be 170cm. I am now 168cm.I am 63 years young.As amkoffee said you are a bit young . I do have a prolapsed disc and degen joints  (osteoarthritis) in my spine.

If you have a muscular issue, as has been diagnosed, I guess it's possible that your muscles are contracting which will shorten your 'stretch'.  posture has a lot to do with stature.  Try yoga and if you havent already done so, stop the chiropractic.  2cm is not a great difference.  In my 20's I was 5'9 and in my 40's I was 5'7.

as stated everyone does louse height as one gets older, but unatural compression at a younger age can be a cuase for concern - so it is best to ask your GP about the issue. 

I know for myself I have gone from 180 4 years ago to 158 as of the start of this year, as in i've lost 22cm in 4 years time do to decompression of my spine to do Spinal Ostioartritis and i'm only 29. 

I think compression of the spine can make a small difference. My guess though would be that you've developed curving of the spine. 'Muscular problems' essentially means that your muscles are out of normal balance. The muscles are supposed to work together in groups to keep the skeleton properly aligned. When, through overuse/bad posture etc, one muscle in the group gets weak and lazy the other muscles over-compensate. Your bones are pulled out of normal alignment. This leads to pain in the muscles due to stress and/or pain in tendons and ligaments due to too much strain being placed on them.

If your spine is pulled out of alignment that creates scoliosis - sideways bending. If you think about it - a pipe cleaner with kinks and bends in it will appear shorter than one of identical length that's completely straight.

Try Googling for MFR or trigger point treatment. There's a good chance that will give you symptomatic relief from the pain. To straighten out your spine and put a stop to the pain permenantly you'll need to address the muscle imbalance and restore normal posture and movement.

I commented earlier but I had one other thought. You will actually be shorter at night then you will when you first get out of bed in the morning. Be sure you measure yourself at the same time every day.