You mentioned that you fully breastfeed your baby since birth: is it 100% breastmilk or do you sometimes supplement with formula as well?
For me, my menses came back regularly two months after birth but I breastfed/expressed milk about 80% of the total milk intake for my baby as I did supplement with formula.
Breastfeeding does not stop the menstrual cycle 100% for some women and is also not a safe method for contraception either. Every mother has different BF techniques, hormonal changes/levels and it is quite usual than one gets menses earlier after birth. If you need better diagnosis, please do consult your gynae just to be sure you are ok. I presume your lochia discharge after labour has since stopped and there was no labour complications?
You can read some info here:
When can nursing mothers expect to get their period? | BabyCenter