I am having shortness of breath without wheezing.

[fanana27]

I've been having shortness of breath for around 2 weeks now. I have asthma history. But there is no wheezing sound. I just feel that i cannot get enough air and i can only get enough air sometimes by breathing in by yawning. I visited the doctor two days ago and they took a lung x-ray. The x ray was alright. I was put on a stronger dose of Seretide inhaler. It does not seem to be working and I find it so hard to breathe properly. What can be the causes of this? I am really at a loss of what to do.