Why is my dog doing this?

She is a 1 1/2 year old Jack Russel. The adoption lady potty trained her. She was good the first week, of course the first couple days she was too busy looking at the new surroundings outside than to pee, but she got it right quick. Just yesterday she has been pissing herself every time we put the collar on her. This is weird behavior and she is acting like she is scared of the collar but before she was so excited to get it on that she wouldn't sit still. We aren't making her overexcited to go outside, and telling her no just makes her possibly pee more. I'm worried about her behavior. Any advice?