While doing seated calf raises, developed lower back pain, what?

?strain/sprain: See how things go over next 3 or 4 days, as a basic strain in the lower back should resolve rapidly unless re-injured. If persistent, might be of value to see your doctor and possibly get imaging studies. A physical therapist could help turn this around.

Nothing to worry : Sometimes when doing calf raises the opposing flexion and extension muscles of the back contract together to create spine stability. A slight asymmetry or mis-timing can cause a minor injury to the back. The best thing that you can do is to regain full activities as tolerated. Note: it is ok to have some pain when increasing the activities.