Suggest Remedy For Delay In Periods

Hi, I'm Paris and 17 its been a week now and I've missed my period, i was on sum medication before which was 4 an eye infection I had, but I stopped taking then about 2-3 days after I was ment to come on.. ( I had ran out ) I'm not pregnant as I've took 2 pregnancy test both say negative I'm still worried, and I'm planning to take more test every time I go out...do you no any other reason why I haven't came on,O and I don't seem stressed, i am sort of now because I haven't came on.Thank You