OK, first, I agree with the others who say that nurse is misinformed. My periods (entire body actually) were erratic for MONTHS every time I had a baby. But listen closely to me now that I have said that!
My second son was conceived DURING my period, or while I was using a diaphragm. The best we could figure, as we had started "trying" for a second baby in August, and I didn't get my period that month, and sure enough was pregnant. We thought he'd be due in mid-May around my first one's b-day, but his due date was mid-April, and my OB pinpointed time of conception to the week of my period, when we didn't bother using the diaphragm. So you CAN possibly get pregnant while on your period. Not likely, but not impossible either.
Also, after he was born, I nursed him for over a year and never got my period back. I had weaned my first son (when I dried up) at 4 months of age, and got my period at the 6 month mark. With the second, even though he was only nursing in the morning and at bedtime at a year old, I was concerned that my cycle had not resumed, but didn't think much of it. At that point in our lives, we were so exhausted and so off-key with each other, that there wasn't MUCH reason to use birth control, so we just used condoms on the rare occasion that something DID happen! :) But by the time I was getting ready to try weaning my son at 13.5 months, I started feeling something hard in my belly. I thought I had a tumor or something and pregnancy never crossed my mind until I felt the baby move! Turns out, I had gotten pregnant with my dtr when my son was about 10 months old. I WOULD have gotten my period then, but never got it due to getting pregnant again! Then she was born 2 months early, so I only knew I was pregnant 3 months before she was born!
So my three kids are a living testament to what God intends to give you! I only have 2/3 of one ovary due to cysts I had as a child, I was on the pill when I conceived my first, the diapragm with my second, and just barely doing anything and using condoms with the third! I cried and thought I would never be able to manage 3 such small kids (my oldest turned 3 in May before my third was born in Sep), but I got lots of help from family and friends, especially since I ended up on bedrest at 6 months with my dtr. Very hard to chase around two active toddlers when you are on bedrest! And life IS hard when they are all little, but it does get (a little) easier as they grow up together...
You may not end up even being pregnant, but please know that if you are, this was part of God's plan, and after having two sons, my dtr has been a true blessing. If it had been up to me, we would've only had the two boys, and maybe adopted a little girl down the road, but she surprised and delighted us all and still does! :D All of my babies (now 12, 10 and 8) are the best unexpected (and unasked for) gifts I have ever received!
Wishing you the best!
A.