Was told I had a bladder infection today. 12 hours later I now have a tight pain on my back, left side half way up. Kidney infection?

Kidney infection. Kidney infection (pyelonephritis) generally causes fever, chills and very significant flank pain. One generally feels "sick". Bladder infection (cystitis) rarely causes systemic symptoms. Discomfort in the pelvic area and irritative voiding symptoms are typical.