My "lmp" Was August 5th But It Wasnt A Normal

My "lmp" was August 5th but it wasnt a normal period it lasted 3 days and started off as brown spotting then light bright red blood but not period blood!. I had sex with my now bf unprotected. Almost everyday that month .one night aug 20 i was intoxicated and hooked up with somone els but he could not get hard and did not cum . i found out i was pregnant 2 weeks later . my first us was sept 23 and read 6 wks 3 days they said 7 weeks 3 days from lmp. I got another us on oct 23 that said 12 weeks 1 day and i was supposed to be 11 weeks. Need help!