Hi, I'd like to help you with your question.
There are several possibilities. One, your mother has something going on that is physical. You mentioned she had testing. That is a good start. Has she also had a blood test? Low sodium levels or electrolyte levels could cause a change in a mental status and physical well being. She may need further screening.
Another possibility is a trauma of some sort. With older people, it is common to not discuss bad things that happen. Your mother may have experienced a shock of some sort and is not willing or able (mentally) to discuss it. It may have frightened her so much that she feels safer inside her home.
She could also be experiencing anxiety in general. People can develop fears for no good reason at all. Your mother could have become fearful and developed a condition called Agoraphobia which is a fear of open spaces. The good news is that it is easily treatable with therapy and if necessary, a mild anti anxiety medication. If you feel she has this condition or you think an evaluation is warranted, contact her doctor for a referral to a therapist. Or you can search on line at http://therapists.psychologytoday.com/rms/.
Your mother could also be experiencing a sudden realization of her age and frailty. In some older folks, they begin to view their advancing age and inability to do what they used to do as something to fear, since it means they are closer to death. While they are fearful of their advanced age, instead of trying harder to maintain their independence they "give in" and start retreating from life. This is often caused by a milestone birthday or other significant event.
What you can do is talk to your mother about these possibilities. Also, partner with her doctor to seek an answer to what is happening with your mother. Screen her for any physical issue first. If she is fine, then try a mental health evaluation. If you can locate a therapist who specializes in geriatric mental health, all the better.
Reassure your mother that she is fine. Offer to go out with her as much as your schedule allows. Invite her over for dinner. Encourage her to become involved with volunteering (preferably with younger people such as at a shelter). Anything she can do to engage outside the home will help her feel better.
I hope this has helped,