Came on my period three days late and went off early and then saw brown blood the next day and then it went away am i pregnant why would it do that?

It happens sometimes: Many women have occasional variation of their mucus discharge, some spotting between periods, a heavier period, a lighter period, a longer or shorter period, or even a missed period. Having one or two of these symptoms doesn’t necessarily mean there is a medical problem, but a visit to one’s doctor is a good idea if symptoms are recurring. Probably not pregnant, but one can check a urine test.