My dd (2y 8m) has been potty trained for over 6 months now and we've not had any issues or accidents (other than a case of cystitis this summer). But recently she's started holding back her poo for several days. She went 6 days until we had a very stressful (and backed up child) last friday. After 3 trips to the loo and about 45 minutes I think she managed to lose half her body weight and was much happier.
I suspect part of it is that she's like her dad and has a preferred toilet and doesn't really like going number 2 on the 3 days she's at nursery school. I started introducing daily cups of orange juice, but don't know if there is much else i can do. I think she's holding it it because she either doesn't want to go at that minute or because she wants to be on her fav toilet, and things just compound from there. Then it's a bit painful so she holds it in more.