?

So today I was asked to pick up my child from nursery by his mother, me trying to be the civil nice guy said yes. I went to his nursery to pick him up and found out I'm not able to as she said said so. Is she allowed to do that? I am on his birth certificate and haven't done anything that should make them concerned for him when he is with me. I have my children 3-4 days a week and 2-3 nights. Thank you in advance, she's really making me feel like such a bad parent but i try so hard for my children and need to get educated on my rights so I can finally put this to an end.