Period is late by 7 days

Sara

My period is late by 7 days and the last time I had sex was in June 11. My boyfriend is currently working in northern California so we are in a long distance relationship right now. I had my period for June and July but somehow August hasent came yet. What could this mean? I can't be pregnant because it's been 3 months away from bf and I haven't slept with anyone else.