Hello; I am 18 days late for my period. Two days ago, I took two home pregnancy tests, both were negative. I had my normal period, on time, last month - and have not had sexual intercourse with a male since. I am experiencing nausea and headaches (although I get headaches every day regardless.) I recently have been extremely stressed, and have flown within the last month for travel. I am not urinating more than usual, but I am very tired. Any suggestions or thoughts as to why my period may be late, but I have two negative tests?