Why can’t I get pregnant?

I am 21 and I am madly in love with my boyfriend. And before I get judgement I know I’m young but this is what I want in life. I am ready to have a child with my boyfriend and I have been trying for the past three months and nothing has happened. I have been recently experiencing irregular periods and I don’t really know when I’m ovulating. In my last relationship I was having unprotected sex all the time and I never got pregnant and I was having regular periods. Sometimes I have pregnancy symptoms but the tests all come back negative. I’ve been to the doctor and she said nothing is wrong fertility wise. Could you guys give me advice to why I’m not getting pregnant.