What happens when a testicle swells up really big & a month later it's flat an small like it's gone?

Hydrocele. Is the swelling completely gone and is the testicle back to normal? If so the most likely thing is a hydrocele which is just a fluid collection around the testicle. The most common cause is trauma or an infection of the testicle and it is completely benign. If it gets big and small and big and small this is a communicating hydrocele or a hernia and would likely need surgery to fix.