Thanks for writing.
You can wait for upto ten days after stopping regesterone
The reason behind no periods could be that you are feeding. The severity with which menses are affected depends on the frequency of breast feeds you give to your child.
As you say you had a protected sex and your urine tests are negative, so pregnancy
should not be the reason of your delayed period.
However before using any hormonal
tablet like regesterone, an ultrasound
is a must.
Now since you have already taken the tablet you can wait for ten days. If your periods do not start then you have to get an ultrasound to rule out other causes like ovarian cysts and pelvic infection