Thanks for Healthcaremagic,
It is normal not to have period during lactation
or when you are feeding your baby.
You got your period 5 week ago, and now you have missed your period if you had unprotected sex during this period we need to rule out possible pregnancy
This is good that your urine pregnancy test is negative. You should take another pregnancy test, or you can do Beta HCG to rule out pregnancy. You can consult Gynecologist and go for transvaginal ultrasonography
It is possible to become pregnant while feeding because ovulation can occur in between so it is advisable to use some kind of contraception
such as copper T, IUCD or barrier methods
Hope this information helps you.