So_embarrassed, All pregnancy tests are not created equal. The modern early detection kits (like First Response Early Result (not the digital one)) can detect sufficient hCG as soon as 8 days after ovulation/conception.
After 3 weeks a pregnancy test should be positive although it is not guaranteed to do so.
Do you have any reason to believe that Mr June7 did not have a vasectomy and lied to you? Part of the vasectomy is to go back and have them check if it was successful. The failure rate is EXTREMELY small. It will be very bad if this guy lied to you - but then he did have sex with you while knowing you were in a relationship so I suppose it would not be surprising if he lied.
Did you allow your boyfriend to ejaculate inside you?
Once you have a dating scan around week 6 of your pregnancy, you will get a much clearer and more accurate picture.
Best of luck!