Counselling on nonobservation of periods

I write to you on behalf of my female partner(Age-23). We are sexually active and my partner has never missed her period. The average duration of her menstrual cycle since one year is 32-33 days and the longest it has been for 37-38. Her periods last for an average of 3-4 days. In the month of December ,2020 her periods began on 24 and ended in 28th December,2020. In the month of January,her periods had began from 24th January ,and we had unprotected sex(ejaculation inside) in 25th. Her periods ended on 29th January. We eagerly awaited her day of periods on24th Feb,2021(expected date) ,but it did not come. She tested herself with Preganews kit on 27th Feb,28th Feb and 1st March of 2021. The test was done in the morning (first urine of the day) and all 3 times it came negative. Yet her periods did not come as of 1st March,2021 - 11pm. Can you please suggest our next course of action because we are anxious. She was also stressed with exams in the month of Feb too. Can stress cause this?