Period during pregnancy

Okay this is the second period in a row that has been late by at least 2-3 days I'm late 3-4 days currently for this months period. Could I have been pregnant last month and not know it. Keep in mind I have had no pregnancy test or anything of that nature cause I didn't suspect it. So I guess the question is, has anybody's period changed during pregnancy if you had a period? Could I be pregnant?