I have been feeling week since the past 20 days. I'm also suffering from stomach ache at regular intervals. Is there somethin serious? My doc said no.

Could be serious. You need an evaluation by a gastroenterologist or someone capable of doing a GI workup. It could be very simple and benign, or an ulcer, or something much more serious. 20 days of stomach pains need to be evaluated... Verify that your doctor fully understands your worries and complaints.