This is very common to have constipation in baby who are on breastfeeding. If baby is comfortable, no abdominal distension, feeding well then no need to worry. You can use glycerin suppository if baby feel uncomfortable. For information you can watch this video on my channel
https://www.youtube.com/watch?v=PRxVEo4xc3s&t=168s
Next Steps
If baby shows abdominal distension, not feeding well, lethargy , immediately consult your paediatrician