Hey ladies!

So I haven't had my period since January and it's already the end of March. I know right away everyone is going to say "Oh you're pregnant!" But I've taken 3 pregnancy test and all said negative, I took them about 2-3 weeks ago. I have scheduled a doctor's appointment but unfortunately I have to wait until April 11th.. so really my question is, what do you ladies think could be wrong with me? Thanks!