You live on your own with your mum and have done for a long time, and you are now at the stage where you are moving away, she might just be very upset that you are growing up, and she will be alone from now on.
Sounds like she is scared that she will be alone. She had to be forced to sign the forms that meant you would leave her.
Perhaps you need to have a chat to her, and discuss how she is feeling. Explain that you are not going away for ever and will be there for her when she needs you, she only has to ask.