What Causes The Contraction Of Penis Towards The Testicles Post Intercourse?

I m having a problem with orgasm and ejaculation. Basically, when I come now, it doesn t feel completely satisfying like it used to. The semen just drips out, instead of shooting out like it used to. Also, after I come, my penis involuntarily contracts for about an hour. It contracts in, towards my testicles. About me: 18 year old male, good physical/mental health, no medication, masturbates only with underwear on