Fastly Breathing. What is the problem?

Sir, My sister is facing the breathing problem every year from last 3 years. After 15-20 days she gets normal.Docters use to say that she is okay and she has nothing. All the medical checkups are clear. The doctors of HINDUJA hospital (mumbai) also that she has nothing. So i want to know that why this happening with her. When the problem persist, drinking even water becomes a big job for her.She started breathing very fastly. If she walks just for 2 steps,she started breathing very fastly. This happens all the time when she is eating,drinking,going for toilet and any shortest or lightest physical work. Today, now she is forced to sleep all the time to get relief little bit. Please tell us sir, what is the problem?