Oxygen Levels dropping

I am a 25yr od female who has Asthma and is taking seretide 250, salbutamol and Ipratroplum Bromide
In May 2008 i started to have trouble breathing i could and can only take short shallow breaths and have chest pain, i was seen by the emergancy doctor and told i was having a asthma attack this was in may and i was prescribed steriods.
on june 2nd i was taken to hospital as my heart was racing and i was still struggling with my breathing i have various heart and lung tests and they first thought i had a blood clot on my lung, but i was later discharged and told i didnt have this.
i had to do a walking test while i was there and my oxygen levels were dropping as i walked.
i have since been on anti biotics as i had some crackling on my chest, im still struggling with my breathing and having to have all windows open and a fan to help with my breathing.
i saw the respiratory consultant a month ago and had more tests done which came back ok except my walking test where again my levels were dropping  - they havent told me what could be causing this and my family and myself are getting concerned about it does anyne no what coud be causing this?
my best peak flow is normally 500 and i have been between 250 and 350 since may
ive also had a cough for about 4 months which is worse in the mornings ive ben told my asthma is getting worse but this is it, im due back with the consultant in oct.