Without contrast in CT scan is it possible to mix up tumors and kidney stones? I had no back pain but was told I had a large stone during a UTI at er.

Not usually. Kidney stones have a characteristic appearance of solid calcified masses in the central area of the kidney where urine collects. Tumors, while sometimes partially calcified, mostly occur in the outside part of the kidney called the cortex and have a prominent noncalcified soft tissue component. When in doubt contrast is used but the need for contrast to tell stones from tumors is uncommon.
No. Kidney stone and kidney tumor are different on CT scan.If the stone is in the kidney pelvis and not causing obstruction it will not give pain. If you have a large kidney stone I am sure the ER doctor gave you instructions for follow up care.Good luck.