I think you should gain courage and tell your mum about your pageant invitation. You cannot tell how she will react about the news unless you tell her. You can convince her, if she refuses to give you permission to attend, how bad you like the invite.
tell her immideantly now!!!!! just take it slow at first and say "mom i was invited to something and we need to have a talk about it pls" and then take it one step at a time and then once she agrees to not be mad at u then tell her