Hi, my period is 18 days late, which it never skips.i took a pregnancy test and it negative.lately my allergies have grown worst.i don't understand, pls?

Possibly pregnant: Urine pregnancy tests while quite accurate depend on the particular hormone being in the urine. This in turns depends on the level of hormone in the blood [varies person-person and length of gestation] and how the kidneys are functioning. Many pregnant women have neg urine for up to two months. A blood pregnancy test is completely reliable. Perhaps pregnancy needs ruled out.

Do not worry: Sometimes this can happen without any serious reason. If the situation repeats( you are not getting a period over 2-3 months) go see a doctor. If your period returns you should be fine.