One day period that lasted less than 24 hours with no cramps. negative pregnancy test taken immediately after that day. what could be going on?

Ovaries: Most likely the ovaries did not work properly this cycle. That can be from stress, other hormone problems, or infection elsewhere in the body.

Period: One can miss a period or have an irregular period without having to be pregnant so just wait till your next cycle occurs

Normal menstrual: variation. The amount and duration of a menstrual period is highly variable and depends on hormonal fluctuations. Stress, illness, medication, travel, etc. all can affect. In many cases a short period may be followed by a short cycle (period comes a bit early next month) or longer period with more bleeding next cycle. I would also repeat the pregnancy test in a week if no still no bleeding.