PLEASE HELP

Marilyn
My periods have been coming late, but this month it's super late. I'm 18 days late. My last Period was on the march 20th and was expecting it to come around the 20th of April. I've taken 3 pregnancy test, one on the 28th of April came out negative, another on the first of May, came out negative and today I took another one and it came out negative. I was under a lot of stress the last two weeks due my exams and school life. My doctors also presibered antipressions recently and I've taken about 5 of them so far. I've had sore breasts since around the 25th of April and still currently do. I get cramps but they go away. Should I be worried about being pregnant though all my test came out negative?