When one is sexually excited, there are secretions that are released from the local glands present in the external genitalia. This is normal.
Pain in the back after urination may be due to obstruction due to a stone or calculi.
You also seem to have erectile dysfunction
. Good communication with your partner is required for this. Also, remove any misconceptions on your sexual life.
It is wise to consult an urologist
for all your present concerns.
I wish you speedy recovery.