Plan b + late period how much is normal?

My period has been late for over a week now. I took a pregnancy test 2 days ago but it was negative. I know plan b can mess a bit with the cycle but I thought it's a few days max. Should I wait and take a second test or better to check with a doc right away?

Updates
9 mo
I went to the doc today and made a blood test. I should get the result sometime tomorrow. He said it's a 50/50 because it happened during the ovulation phase and the plan b I took only works if taken before the egg is released. The symptoms are very common for both plan b and pregnancy, I'll have to wait for the result to know how sure
Plan b + late period how much is normal?
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