Pft are normal but still experiencing shortness of breath. what can be the cause? also been having lots of mucus for a couple of months. gerd?

Many causes: Anemia, metabolic acidosis, thyroid excess, pulmonary embolism, pulmonary hypertension, hyperventilation, and panic attacks - none will cause abnormalities on PFTs. Other causes will but may be minor enough that you've been told they're "normal". Moreover, some won't show on spirometry alone. You may need diffusing capacity or provocative tests to uncover them. Bottom line: you need more work up.