Super late period??

Ok so my period is now late by about 23 days. I've taken two pregnancy tests this week that were negative, and from the last time I had sex (late April) if I were pregnant I'd be so far along that there's no way it'd be inaccurate. Plus I had two normal periods since that sex, and took a plan b the morning after. Before that I was taking hormonal birth control that delayed my period until March. I'm not sure why my period is this late, could it be leftover effects from my birth control and the plan b? Is it stress (I've been stressed lately, but stress has never affected my period before)? Is it worth it to go to a planned parenthood and have them do a pregnancy test/ask them?