I have back pain everytime I do squats. Why?

Unknown. Squats put a lot of pressure on the intervertebral disc so this is a possible cause of the pain. Facet joints are also a common cause of pain. To determine the precise cause, see a pain specialist. He/she can anesthetize (numb) each structure one at a time until you report relief of the pain. Then a treatment can be targeted for that specific cause.