Is it reasonable that i get mad at my husband for helping his ex wife with their son?

I work full time and have 3 kids of my own. my husband has one son with his ex wife. we now have custody of him but now even on her days to pick her son up she makes excuses. saying shes very tired from work and school and if he can help her by picking him up. ahe has no other kids and is not married!! it makes me so mad.