Hi ! Possibly your testis has undergone unnoticed repeated trivial trauma during the sexual acts. This has lead to the inflammation of the testis and epididymis
due which it is enlarged and feels heavy. Prolonged standing causes the enlarged testicle to drag the cord structure causing pain in the testis which may radiate to the groin and also to the abdomen. This is negated when you lie down and so the pain subsides.
You have mentioned about milky substance coming out. I am not sure if it is infected substance or not. You have to get it tested in the lab for routine and culture sensitivity test, along with a urine test as well.
Till that you should avoid prolonged standing and lifting heavy weights, wear proper scrotal support, go for some anti inflammatory drug and antibiotics depending on the culture sensitivity reports. In case it does not help you, you should visit a urologist
for physical examination who may advise you for an ultrasound
of the testes, and other necessary tests.
I wish you an early recovery.