Hospital visit tests were negative but still having symptoms

Had some bad cramps and feeling very sick and tired. No tender nipples though. Went to a and e 3 times and they took Urine samples and confirmed negative pregnancy status all three times. I also had a short period last week which was brown and light. Still feeling very tired and a little nauseas and having some cramps. Could it be pregnancy? Can I still get a positive result after having negative ones? Or would it shows positive straight away?