My last period was in june. we used condoms and i have 3 negative pregnancy tests. my period was supposed to start before we had sex in july, but i missed. i am worried?

You'll need to repost with more specifics. Were your three pregnancy tests performed a week after your missed period? Did you use condoms in your July encounter? Missed periods can be caused by a number of different factors including pregnancy. Since you've missed July, Aug, and Sept, take a test now. If negative, then see your OBGYN for further evaluation.