What is the problem

Am 21 years old. My period is a month late now.. My last period was on December 4th 2019. So I was supposed to see them again on December 28th but they didn't come, I was then supposed to see them on 21st January, well I didn't... Av got two late periods... Previously on Dec I had taken plan B but am not sure if its causing all this delay... I taken 5 pregnancy tests at different intervals but all are negative what could be wrong