My Dog is All of a Sudden Terrified to be at Home

Question
I adopted a mutt from a shelter about 4 years ago. She is a 5 yr old Hound, German Sheppard, and Retriever mix. She has never really been afraid of anything. Espcially at home. I came home from work the other day and found her in the bathroom with the door shut (this has never happened). She has been panting, shaking, and not leaving my side for a two days now, and it gets even worse when I have to leave or when I come home.

The only thing I can think of is that it might have been a noise, but this is not really plausible. I live in Chicago in a 1 bedroom apartment, so she is pretty numb to any startling noises. I even checked with my landlord to see if someone had to come in unexpectedl and may have scared her, but he said no one has been in there.

I don't know what to do at this point. I want to console her, but I feel like this may lead to bad habits every time I have to leave, and she has to be ok being on her own during the day. Some people suggested a thunder shirt, but I feel like this is a temporary solution to a bigger problem. I'm trying to get her back to normal. Any ideas would be helpful!

The door opens inward, so it is possible she could have closed it on herself. I guess my question is if that was the cause, why would she continue to be afraid of every little noise once she is out for a few days? I could see it causing her to be scared of the bathroom then, but not the whole apartment.

Answer
There is really not enough information available to determine what the problem is. You have been very detailed but even you are not sure what has caused this event. I would recommend putting a camera where you can record what goes on and then see if there is something that triggers her. You may also want to consider letting someone sit with her or taking her somewhere to board her for a few days while yo are at work.
Making sure that she does not get locked in probably will help but I would say to take her in the bathroom and close the door to show her that there is no threat.