It's highly unlikely that you would conceive the day after or the last day of your period. This is because to conceive you would need to be having intercourse around the time of ovulation - which is usually mid cycle. So the day after or last day of your period is usually way too early for ovulation.
Like said above, the Plan B pill can mess up your cycles and is actually pretty bad for you body. This causes heavy and irregular bleeding, cramping, nausea etc so looks like everything you're experiencing is due to that pill.
Be aware it could take a while for your normal cycle to return because of taking the pill.
Plan B is only effective up to 72 hours (3 days) following the sexual event and is more effective the sooner you take it. If you took it within that window, you are most likely not pregnant.
The heavy bleeding with clotting and nausea is more likely to be Plan B messing with your cycle, which it can do for a while in some women.