Late period 4 days as calendar says usually very accurate took 2 test both negative. what can it be?should i stop thinking about it stress taking over

You can retake: home pregnancy test in 4 to 7 days if still no period. Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (i.e. PCOS). #1 reason is

Late Period: periods can be late for a number of reasons. Stress, infection, hormonal imbalance, thyroid disease and of course pregnancy. Sometimes it just occurs for no apparent reason. I would have you check another test in a week, either a home test or a blood test in a doctors office. If you then have a period next month things have straightened out, if not, time to see the doctor.