Spotted 18 days after period then period was 4 days late im a 26 day cycle always never spotted before what could it be? Negative pregnant test

Breakthrough bleed. You may have had an annovulatory cycle. This simply means you may not have produced an egg for the month and so your uterine lining got thicker than usual and then broke off a bit (spotting) and then the late period. Its not uncommon. If it persists however, go see your gyn and get some blood work to rule out a few things like low thyroid and get a blood pregnancy test as well.