4 months!?

Danielle
Hey guys so im kind of confused here 
I been having my period for the last 3 months regular(May,June and July) the last time i had intercourse was in may 13th after that i took plan B and 5 days later i went to the gynecologist and took a pregnancy test and they told me it was negative, now skip to august this month todays the 6th and my period was supposed to be between today and tomorrow i have been having crampe and my boobs are sore kind of freaking out in all honesty that i could be 4 months pregnant im 18! So im really really worried because it was my first time and im just really stressing out
Thank u for u guys advices and comments! Some positivity and good vibes could help!