Constipation

We’ve had some random constipation in the last few days and I have no idea why! We haven’t started weaning as he isn’t ready yet, nothing had changed in milk amount and he is fine on himself. Anyone been the same? I was changing him yesterday and he was straining so hard, a firm-ish lump came out and then loads of regular texture poo behind it! The straining is still happening today but nothing as yet!