Why can't i produce Cum? I am 13 years old and like to masturbate

Okay so atm I am 13, I have hit puberty fully but I still cannot cum, All I can do is pre-cum atm. is their something wrong

when I am alone I like to touch myself but I can only go for about 4 mins then I do not want to do it any more, but I can go back in 30 mins

I feel like this is a problem and I am to embarrassed talk to my Dr.