Period 4 days late, 2 negative hpt, no stress in life, not taking any medicine, healthy diet and exercise. why is period late?

Late period: Hi. HPTs may not reflect pregnancy within 4 days of a missed period if you are pregnant. If not pregnant, there can be myriad reasons for variations in periods. Wait another 5-7 days and repeat the HPT to make sure. If pregnancy is not desired (and at your age I sincerely hope it is NOT!), might I suggest you talk with our good friends at Planned Parenthood about contraception. Good luck!