Breastfeeding moms or any moms

So i was talking with the ladies at work. Most of them were on their period and I mentioned that I haven't gotten my period because im breastfeeding. (My son is 5m old) They said it isn't normal to not have a period and said i should get checked out.  
Ps. I havent had sex since my son was born. Ive only had my 6w post partum check up I haven't gone since