infertile?

i’ve posted here a couple times recently. my period is 7 days late and my last one was early. but i took a plan b back in late november. i really want kids in the future. but i’m afraid these are signs of infertility. my periods are normally fairly regular. (im 21) i’m definitely a hypochondriac and worrying about all possibilities. i’m also not pregnant. i took 3 tests all negative. i have an appointment in a week about this but i’m really afraid of infertility. me and my boyfriend of 3 years really want kids and it’ll crush me if i can’t.