Pain in lower abdomen and left testicle with rapid scrotum expansion and contractions, can it be something serious?

Depends : Did you have any trauma? You really need to see your doctor fro good exam. Sound like you could have pulled or strained. Good area for a hernia. Need more details please

Emergency: Go to ER ASAP as could be torsion of testicle which can be dangerous.

Yes: You need to see your doctor. Young folks your age can get testicular cancer, especially if the problem persists. However, can also be a hernia or something else benign. If gets better after pushing contents back in, it could be a hernia. Still need a doc to examine it. Could also be a cyst.