Doesn’t sound like anything too serious to me - as guys, we sometimes obsess over things like penis size, testicle size & appearance, etc. It’s somewhat normal for testicles & scrotum to change size, shape & color during or after sexual activity. Normally there’s an increase of blood flow to that whole region during sex, also, the testes kind of ‘tighten’ & pull up toward the abdomen when a guy is sexually aroused & then will drop back down afterward, so that may be what you’re seeing. Keep your appointment w/ your PCP, I think he or she will likely agree that there’s not much to be concerned about.
Check back here later & let us know what you find out about this...