Can lower back pain (lumbar) be cause by not stretching your calf muscles after soccer?

Maybe: Depends on how you're stretching your calves. If you're stretching calves and lower back at the same time then lower back may have been neglected. If you're standing on a plank, and doing calf raises, lower back should not be affected. The other question is are they innerveted by the same nerves? Answer is no.

Unlikely: Not likely but tight muscles at the hamstring level and/or hips can be a sign of a spondylolysis/spondylolisthesis and flexibility maintenance makes for a healthier back.