In need of advice

The last time I had sex was June 24th and June 25th. I did have a normal period on July 8th through July 12th. Now I am a little over a week late for this months AF. I have taken 3 at home tests and all have came back negative. Any suggestions or advice as to what could be going on? My boyfriend has been away with the military since May 3rd & is coming home next month. Could it be possible that the stress & excitement of him coming home is throwing me off this month?