Thanks for query,
Size of penis in normal condition is small but when you have arousal blood supply to it increases causes its size to increase.
Sex is in our mind & not in organ.
Tension about small penis may resulted in to ineffective stimulation to parasympathetic system leads to this situation.
Just leave all worries of sex.
Your pain in that area was examined by Nephrologist
& declared normal so nothing to worry it may be due to pschycological reason.
Just leave all tension you will get sexual arousal as like as early you get.