Possible reason why pregnancy test positive in the morning and negative at night?

Due to change. In urine concentration, i rec u do a blood test does not matter what time of day for a blood test.
Urine concentration. First morning urine is more concentrated and would have a higher concentration of HCG and may give a positive result. Urine later in the day may be dilute enough to produce a negative result. This is assuming that you did the test correctly.