HELP: skipped a period

Lisa
Hi guys,
My period is always on time if not a day early, however this time I am late and am afraid I will skip my entire period.
I've tested on pregnancy twice, and I'm not pregnant. I don't take birth control. My period has been regular for over 10 months. 
Could it be std's that mess with my period, or is it just stress, or I really like a guy. I don't know guys, please help me out from your own experience.
Big thank you!!!!!