What Causes A Delayed Period With Negative HPT?

so my boyfriend and i have sex but use a condom every time and for some reason i have stressed about possibly getting pregnant all month for no reason. I was late for my period (but have been feeling funny-possibly from pregnancy or could be due to me stressing), and on the 2ND missed day i took a hpt and it was negative. Should i buy another one in a few days and try again, or has me stressing about it all month affected my menstrual cycle to the point that i will fully miss it this month.