I had unprotected sex two days after my period then took a plan b pill the next day. i haven't had my priod it's almost 3 weeks now. i have taken 7 negative pregnancy tests, two of which i took at my gp. what could be the issue ?

If you are 3 weeks late for your period (about 6 weeks from unprotected sex), then a pregnancy would certainly give a positive test by now. The Plan B could have disrupted your cycle and your periods may just need to reset and will resume over the next month or two. If that does not happen you should get additional evaluation as a variety of health issues can also disrupt your cycles.