Helo my last menstruation is January 22 till 25 but in February my menstruation did not come till now I buy pregnancy test but all negative what is the cause of this

Either you are skipping a period, which is common enough, or you are very late, or you are pregnant and it's too soon to test, or you aren't using first morning urine, or you are pregnant and one of the few women who never test positive. (I think that covers everything.)