Asthma is relatively common in cats. It often presents in young(ish) cats and can seem to occur quite commonly.
There is often (but not always) an allergic undertone, so think if you have added anything different to the environment such as a smoker, air freshners, paint, flowers etc.
Xrays are very useful to differentiate asthma from other causes of respiratory disease (in cats this may be lungworm, bronchitis, heart disease etc).
If a diagnosis of asthma is made there are several treatment options, that may include corticosteroids (either orally or by regular injection), antihistamines (orally) or now you can use specially adapted human inhalers.
The latter is by far the most efficatious, although it does require a bit of training and patience for cats to get used to. This method is safest and often most effective.
Usually the condition is long term unless the allergic undertone can be identified and removed.
It is a very treatable condition thankfully and no reason why your cat shouldn't have a normal and healthy life.
Best of luck :)