MEDICAL QUESTION-My partner got memory loss out of no where, almost forgot me entirely

Im not very smart when it comes to anything medical, I need help but I cant really find answers online. If this isnt an appropriate subreddit I will take it down.

Me and my partner (F20) were arguing and almost breaking up for the 3rd time. They packed their things to leave, I felt awful so I admitted fault. After that they wanted to sleep one more night and think about how they felt in the morning. I woke them up and we spent all morning together, then they went to work, they came back . We got food and came home, we ignored the fight completely, we were watching a youtube video and out of literally nowhere they start asking "what happened, im kinda confused where I am (my room)" "are we dating" "whats going on". I didnt answer i thought they were messing with me. I dont answer and I get frustrated because I thought they were faking it to a degree and it was some sort of derealization which i lived in for a minute but i know can go away if treated right. They insist that they are worried because they cant remember when we first met, they don't have memory of us but they know that they love me and were close, I think to talk about it with them after work but they left me a text that they thought i packed their things even though it was themself !! I'm scared there developing Alzheimer's or some sort of amnesia.

They are born female, 20, white, they smoke pot everyday since they were 14, they do already have a bad memory problem and they dont really remember much things they learn ( i remember things that were literally meant for them). Its just so odd that it seems like they got hit with the memory thing from men and black literally out of nowhere, its scared the actual snot outta me.

Im really worried I'm just wondering if anyone could help out. I am more than likely going to try to take them to an emergency room. Thank you for anyone who took the time to read this !

(sorry if my english isnt perfect)