HELP! 11 week old Jack Russell puppy hates collar

Please help if you have any advice. We adopted our Jack Russell at 8 weeks, she is now 11 weeks. She has been a happy bouncy pup until 3 days ago when we put the collar on her. Since then she wont play and she spends the whole day sleeping or scratching at the collar and will only go outside to toilet if we take her out and then runs straight back inside and into her crate with her head down. We are trying to be strong and not take the collar off because she needs to wear one for her safety and puppy class but it is hard when it makes her this unhappy. What should we do? Has anyone been through this before? The collar isn't too tight and is nice and soft so I don't think that would be the problem. I don't know what to do because I feel like it could be creating an issue with her development and her bond with us, but she needs to wear a collar!

Thanks in advance