Numbness and tingling in left shoulder blade?

this has been happening for 3 days now, and it only starts becoming numb and starts tingling when I sit in a straight position, whenever I move my arm to the back the tingling and numbness just stops, but it happens again after a while of sitting in the same position, it doesnt happen when I am leaning back on the chair, any idea why this happens? and any tips to fix this?