Why do I experience intense post-ejaculation pain?

Please visit a urologist. Based on proper evaluation and diagnosis, the doctor will be able to determine the cause of your problem and suggest treatments accordingly. Also, to manage pain on your testicles I advise you to not sit for a longer duration and avoid the activities that worsen the pain. 

This could happen due to infections or skin conditions. The itching on your penis might be caused by fungal infections (such as jock itch) or other skin irritations. Please get it checked

It’s very normal for a penis to be smaller when it’s not hard, and then grow to about 11-12cm long when it is. This happens during puberty which is usually from around  when you are 10-14 years old. So you shouldn’t have anything to worry about.

You might have this problem if you are feeling pain when passing urine if you feel like you need to pass urine more often than usual, or when your urine looks cloudy. A bladder infection happens when bacteria enter the bladder. However, it is not usually serious. To cure it, your doctor might give you some antibiotics that can help kill the germs causing the infection. Ensure that you drink a lot of water to help wash away the bacteria. 

Burning when urinating, that uncomfortable­ sensation, may indicate an infection. Your urinary tract could have­ bacteria invading, causing irritation. Alternatively, a se­xually transmitted disease could trigge­r similar symptoms. Staying hydrated helps flush things out, but see­king medical care is crucial.