It could be the case, absolutely - favouritism and conditional love (such as only showing love towards you when you meet her expectations) are both possible symptom of narcissism. Having said that, it can be incredibly challenging to diagnose somebody without meeting them, especially when you only have two or three core points of data to work with. I would be hesitant to trust a Redditor's diagnosis automatically. What makes me think that this might not be a basic case of narcissism is that your mother's behaviour began 5 months ago. She was presumably less hostile with you in the past, and showed no clear signs that she may be generally toxic, or have any kind of cluster B personality disorder.
If it started when you moved back in, she's most likely feeling tension or resentment towards you - this could be because she's used to more freedom than she has with you living there with her, or it could be that she had certain expectations about what you would achieve while you were not living with her, and perhaps you did not meet those expectations. It could just be that something else is going on in her life, and because you are around her more than most people she's redirecting that frustration (unfairly) onto you.
If there's any further information about what specific behaviour she has displayed, or how she's treated you prior to the last 5 months, it might paint a clearer picture about what's going on.
No matter the reason, it is undoubtedly difficult that you're having to live with somebody who is making you feel like you aren't loved or supported. Depending on your age and other factors, you may be able to seek support from government agencies or non-profits that might be able to get you other residential options (or at the very least, listen to you so that you don't feel like you're in this alone).