Incredibly irregular period

Ever since I started my period when I was 11, it only comes every 3-4 months. I’ve never taken birth control and I don’t have any uterine problems. When it does come, it’s for about 5 days so it’s not like it lasts forever. My doctor isn’t concerned about it and told me that some people never have a monthly cycle, but I’m starting to get concerned that maybe I don’t have enough eggs, and that when I try to get pregnant (which isn’t anytime soon, since I’m only 20) I won’t be able to. Does anyone have any advice?