re: Shortness of breath

Any help would be so much appreciated. Its about my mom who is 88yrs.old. she has been having these shortness of breath attacks for the past 2 weeks. Needless to say she has been to the ER 3 times for this. First they said it was asthmatic Bronchitis, 2nd visit with in 1 week they said Pneumonia 3rd visit yesterday they say she is fine and want to release her home from the hospital. They have done ctscan, ekg, echo cardigram and say nothing is wrong. They wanted to release her which I declined to take her home because of this rapaid breathing she gets just from sitting it gets so bad she can't even talk, and her md and pulmonary say she is fine. It has been very trumatic for her having to be rushed to the ER all these times and then go back with the same symptons now to say she is fine. They give her oxgyen and enembolizer treatment, but no whizzing, no cough, no cold symptons. IM desperate for answer as to way just out of the  blue she gets this shortness of breath just sitting. All the above tests are negative. She did have heartattack 1/1/10 and had 2 stents but has had no heartproblems till the last 2 weeks. I need answer quickly please.