I am sorry to hear about this difficult situation you are dealing with. It does sound very hurtful to you, especially since you cannot understand what is going on.
You did mention quite a few stressful factors in her life like her job, quitting smoking and difficult relationships. However, this usually does not justify someone losing contact with reality and talking incomprehensively like the examples you gave. It is also interesting what you say that she does not remember these incidents after a while. I believe that you would need to rule out any alcohol and drug abuse first before making other assumptions. After that, I would advise you to speak to any of her close friends and family members to see if they have picked up something.
It would be wise if she could get evaluated by her GP and you could go with her for support if she could accept this. However, it is possible that she does not realise that she has a problem and she might resist asking for help. As she is an adult, your powers are limited to possibly asking her friends or family to help you convince her. Chose your time carefully and state your concern that there may be some problem. Try and be understanding to her fears of speaking to a specialist but also explain that this is no life for you either, as it is hurting you to see her suffer. It might help if there were other witnesses to these incidents or something to record her while behaving bizzarly.
I understand that you would not feel like leaving her on her own when she is like that. However, you would need to think about yourself as well and protect yourself from possible abuse. If you feel that this is getting too much for you then you would need to have identified another place to spend a night or a few days away, especially if you have your own flat. If you are very concerned about her during an incident, then you could call an ambulance and they would take her to an A&E where she would most likely have a psychiatric evaluation. You can also call your GP surgery, ask to speak to her GP and explain the situation. Although they will not be able to give you any diagnosis they may give you some advice on how to approach her.
This can be too much for you to deal on your own if it continues. You need to have active support from her family and friends. So don't be afraid to ask for it.
Please feel free to share any feedback on these thoughts.
All the best