sensation of gravity pushing up spine ?

hi, like the title says i am getting this kind of feeling in my lower back that gravity is pushing up my lower back. Its hard to explain its like my low back muscles cant stabalize my spine when i stand up sometimes or sit up straight and i have to put my hand on something to balance myself. Its not painful when it happens more a sort of shacking and seems as if its going up from my lower back.Currently have pain in my shoulder blades that's separate from this sensation.
Brief history of my back problems,
i am a 25 year old male who has suffered with back pain on and off for about five years i have injuried my back on a few ocasions but never been to physio or had a scan on my back.
About a mouth ago I started doing stretches and the ones that pinched my shoulder blades together gave me back spasms in my upper back, i tryed doing the door strech where u put your hands slighty above your head and lean forward and hold for 20-30 seconds i got really bad chest pain and i could not breathe probadly for about a week. i aslo tried lower back streaches after doing all these variaous streches my body as felt awful. any way i am getting this sensation like descriped in the first paragraph and i dont know what it is.
(NOTE) i have never had shooting pain down either my legs or arms