Hi, I Have Noticed That My Testicles Are Rather Small

Hi, I have noticed that my testicles are rather small and I am quite worried that I have caused damage, my penis has reduced in length and I have noticed that my scrotum is also quite flat. I exercise regularly and have not seen any signs of testosterone deficiency, my hair is thinning a bit although it is still growing, I have no pain whatsoever however I have noticed my errections aren't as strong, could this be just a phase? I am 20years old and I am quite fit for my age..