I quit caffeine for a couple of months last year and felt better for it, I remember getting headaches and lower back pain for a week or two while I was going through withdrawals which was odd. I don't even drink particularly much either, 1-2 coffees a day and sometimes none on weekends.
To be honest the strength of those coffees could be more telling. The coffee I had in the morning was a double espresso, I'd estimate around 150mg caffeine. Most people who have an instant coffee will be getting around 50mg.
The lower back pain is normal, this is caused by your bowels getting upset at the lack of coffee and some mild constipation.
I assume your back "using" again?