Constipated baby since starting breastfeeding

[rurally]

Hi!
Formula fed baby from birth, he's now ten days and I've started giving him the breast to attempt to begin combi feeding.
Since doing this he's seemed constipated and getting upset and straining when trying to poo. Is this normal?
How can I help him? Will he get used to it?

Breastmilk doesn't cause constipation. It's very unusual for a breastfed baby to be constipated actually so it won't be due to breastmilk.
If you waited 10 days to try and breastmilk then are you sure your milk hasn't dried up?
It's quite normal for babies to make noises or even cry when they're trying to poo in the early days.
If you're concerned about your baby at all at this age then call your HV.