As you have not mentioned what diet pill you have been taking ,it is difficult for me to come to a diagnosis.Many popular medications for diet like phen-fen are known causes for lung toxicity(although unlikely as these drugs have been off market).
Also women of your age are more prone for asthmatic attack especially if you have a family history of asthma. Asthma occurs due to paroxysmal closure of the lung ducts due to sensitivity to certain environmental or genetic factors.
I would recommend the following.
1) If the symptoms of breathlessness get severe use an over the counter inhaler like Ventolin (Salbutamol).It helps to give temporary symptomatic benefit.
2) If the symptoms worsen consult a pulmonologist for a bronchodilator challenge test, which helps in diagnosing asthma.
3) Get a chest x ray to rule out any other pathology of the lungs like infection.
4) If the breathlessness is more before exercise or in the morning .consider using the ventolin inhaler before exercise.
5) Try to protect yourself from dust and pollen which exacerbate the bouts of asthma.