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. We asked someone we knew that works in a vet office and said to make sure not to make a big deal about it. We did that and she is still peeing. Last time we took her out we decided to leave her collar on. She stayed in the kennel for about a half hour so she new she did something wrong. I just took her out, picked her up and put the leash on, nothing. She peed outside and then we came in. I decided to experiment and took off the collar once inside then put it back on, her tail was behind her legs and she shook a little. I pretended with the harness we use to walk her and she was only slightly anxious but nothing compared to how she reacted with the collar. Like I said she was perfectly fine with the same exact collar before yesterday. Any advice?