Dear friend, welcome to HCM.
this could be tortion (twisting up) of testes, and if untreated could lead to serious problems. it occurs with sudden weight lifting.
if you have pain in back/ tummy, nausea/ vomitting, pain in the testis, and inability to feel the left testis in usual place (left scrotum
sac) by finger palpation (feeling with fingers the testis ball),
you should see a good surgeon and get a sonography
of the testis done.
on the other hand , it may be a simpler reflex pain .
if persistent do not wait longer.
keep me posted on the issue.