Low libido level decreased sexual desire

My libido level has decreased very drastically low..it was a time once when i got excited even thinking about sex...but now my excitement level..my sexual desire is almost finished.. and the shocking think to know is doctor i'm only 19 years old..why am i facing this problem i never masturbate too much...never had too much sex..and never got into any bad habbits like smoking or drinking alcohol or any other drug or addictions only occasionally i had sex and when i was 17 i occasionally sometimes masturbated..till then everything was fine with me..but after that i stopped masturbation process as it was not a good habit...but after 1 year when i first time had sex 3 months ago..everything was fine till then too...but now i dont know what happened why am i not interested in sex anymore...and its only 3 months i'm having sex ever in my life...my desire for sex is finished this type of felling comes in my mind...i dont have any other problem.. my penis erects ..ejaculation process iss also