Constant fatigue, shaky hands, headache, and weight loss

I am a 21 years old student. I have been feeling constant tiredness, weak and shaky hands whenever I perform any activity for more than an hour. Though, it also happens when I’m only sitting doing nothing. Sometimes, I also experienced hot flash, where my body suddenly felt sweaty, heart pounding rapidly and shaky hands when I was just watching tv.I have nausea before and after meals, I noticed it started around March, 2021. I noticed my body weight suddenly decreasing although I thought it was normal, I was at 56kg around Jan 2021 and in Nov 2021 when I just weighed myself it dropped to 44kg. I did not diet, I ate 3 meals a day but still lost 12kg. It actually started in Sept 2020 when I caught high fever and extreme fatigue where I could not even get up from bed. Then, I realized more symptoms rise after I have recovered. I went to see the doctor and he prescribed me with Vitamin B12. He also advised me to go to the hospital if my condition worsens. He also did ECG but at that time the result was okay. I have a history of low blood pressure but never went to any doctor because it was not constant and I don’t know what my real problem is. I noticed my condition worsened because I have to rely on my B12 supplements. It was only after I did a few searches about my symptoms that I found out the best supplement I could consume was this (B12) and it was not even recommended by any doctors I met. The very recent doctor I saw today even told me that she doesn’t know what to diagnose and suggested I go do ECG at the medical centre. She also stated that if my results turned out okay, it could probably be because of stress. So I need other opinion and a better recommendation as I do not know what to do and this condition that I have limits me from doing any activities that I like such as running, driving, swimming, walking or even hiking.Even my daily activities are limited to standing a while before I need to find a place to sit or else I will go vertigo and the feeling of fainting. I hope I can get the right diagnosis for my condition. Thanks.