I understand your concern.
The urine pregnancy tests and ultrasound
have ruled out pregnancy.
Usually the period occurs after 3-10 days after the completion of primolut n
You can wait upto 15 days for onset of periods.
There might be hormonal imbalance or effect of prolactin
hormone post pregnancy.
Nothing to worry.
Avoid stress.Take healthy nutritional diet and adapt healthy lifestyle like exercise daily.
kindly get thyroid profile done to rule out thyroid abnormalities as they might also cause irregular periods
Hope this helps.