Have a look at eric.org.uk I found a page on there the other day specifically about poo withholding. I'm having similar issues with my little boy. It's all started since he started pre-school and I think it's down to the change in his routine I asked at our children's centre the other day and they advised me to keep a little record of exactly what he's doing and how often he is going etc. and then if I call the HV for advice or go to GP I'll have all the information they might need.
My little girl did this - have a look at the Eric website. She poo withheld for about 2-3 weeks - would only go every 4 days or so. She was only going in her nappy so I would put a nappy on her after dinner every evening until she started pooping more regularly, then one day she just asked to do a poop on the potty (as I had told her she could have chocolate if she did it on the potty 🙈) and we made such a fuss of her going she’s gone regularly on the potty without encouragement. I think we may need to get stool softeners as she’s uncomfortable going still but is at least going now! Hope it gets better for you soon.
Apparently really normal, especially if they have had one event of constipation during early potty training. Your HV can give you some tips and can do a referral to children's bladder and bowels team and will probably get you to ask the GP for some laxido x