3 days ago my pet died and i constantly cried whole day , later that day while I was having my online classes, i suddenly had an anxiety attack and started having fear of death , from that time I have a wierd feeling in my head an i feel mentally tired as if i am dying b, when I go to bed i dear if I'll die , doctor gave my chlonazepam and vitamin b12 and vitamin d3 , i  am 15 year old and fear if i have fatal disease or anything . Please help