What is causing my late period?

I took a plan b at the end of January and got my period like a week later which was expected. now my period is 3 weeks late and i took a test but it was negative. it was a pretty cheap test but I heard that doesn’t matter. I have had light cramps, back pain, and have been super tired but those are also all period sunrooms. what else would be causing my late period or should i take another test soon?