Could anyone please help me of why im feeling this way?

I recently had Bronchitis , and took Antibiotics Called Apo- Clarythim .. Im done them and the bronchitis is almost fully gone. There is only one problem ive been experiencing episodes of feeling like im going to pass out and everyday ive been tired and off balance. I find when im in a sitting postion the feeling goes away ... but whenever i do physical activity i get so tired so dizzy and feel faint and then i have to sit down and when i get up again my whole entire body feels heavy.... im going for a CT Scan Plus bloodtest soon but before that i would like others opionion on this.