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.

See below: If the swelling was associated with significant pain and discomfort, you likely had an acute “testicular torsion” which cut off the blood supply to the testicle and results in eventual shrinkage. Severe infection or inflammation (eg mumps orchitis) could also cause similar painful swelling followed by shrinkage. If it was missed torsion, could happen on opposite side. Recommend urology evaluation