Slight Breathlessness and Cough. I do?

I'm 25 year old and I have don't have an active exercise filled life. I weigh around 100kg which made me worrisome. I started of with brisk walking for about 1. 5km and brought it up to 3km in 2 weeks. 3 days back  I have been observing that I feel slightly out of breath at rest. Even a slight chest discomfort. I have this cough at times with frothy kind of phlegm. Sometimes it hurts when i cough. Sometimes it doesn't. Sometimes it if I take deep breath right side of my chest hurts a little and sometimes it doesn't. I always wake up with stuffy nose /runny nose or phelgm in the morning which reduces by noon. I visited the physician and after listening with stethoscope he said lungs are clear and probably due to allergies he gave me citrezine tablets. There's no change in the symptoms I described earlier. What should I do? My BP is always in the range of 120 to 130/80 on BP Monitor at home.