Why do i feel so weak and faint?

im a 14 year old boy with a anxiety disorder,ive been sick for a week but my doctor said i look fine and come see him again in a week if it hasnt gone away yet and everything is probably due to my stress and anxiety, i asked him if its a sinus infection but he said its most likely not. but ive been feeling so weak and faint lately and at times really lazy and dizzy. idk why. BTW when i was sick i was coughing,sound congested, runny nose, feeling like puking but never did, big headaches (like head pressure) i need help ASAP cause this is not very fun