Hi, I Am A Young Male And My Age Is

Hi, I am a young male and my age is 17. I have been doing sex/masturbation atleast three days a week since i was 15. And it was all normal. I got a hard Errection every single time. Even though sometimes i also have done sex in a row 2-3 times a single day and i got errection every time even after i just got discharged. Sometimes, when my sex partner got changed i feel some difficulty in having errection maybe due to nervousness. But from last 3 days i am not having proper errection. I got errection but its not enough hard to have sex and i keep losing it again and again. I am very tensed that what is happening to me and i need your help. Please reply me as soon as possible.
Note: I am medically all fit. I dont have any heart problem also i dont have diabetes or any other disease and i am not taking ay kind of medicine. And also i dont smoke or drink. Please guide me what to do..
Thankyou!