When I worked in OB, we had a patient come in who was exclusively breastfeeding her 9-month-old and was pregnant. I'm not exactly sure how far along she was, but the only reason she came in was because she was already feeling movement. She was heavy, so her pregnancy wasn't apparent just by looking.
Yes I got pregnant when my twins were 6 months old.
They were not sleeping through the night or taking any solids or pacifiers.
they did get one bottle of expressed breastmilk a night (to take medicine) and I got my cycles back at 8 weeks.
I had gone through two years of IF treatments ending with IVF/ICSI (I had PCOS and dh had extremely low sperm count) and we had only DTD a couple of times since the twins were born so to say we were surpirised is putting it lightly even though my cycles were back.
Even if you are not menstruating and not giving any solids or artificial nipples and your baby is not sleeping through the night you will need to watch your body's fertility signals (like cervical mucous and cervical position) to rely on breastfeeding as birth control since the longer you go before getting your first PP AF the more likely it is that you will ovulate before getting it.