This could be asthma and you should be checked for that, but this sounds more like anxiety attacks, in some cases referred to as
panicPanic disorder
Panic disorder with agoraphobia attacks. At age 18, this is very unlikely to be serious disease of your heart or lungs, but at minimum, you should have a physical exam of your heart and lungs, a simple breathing test called spirometry and a chest x-ray. If these are all normal you should discuss the possibility of this being anxiety, with your doctor, and consider a trial of an anti-anxiety drug. Anxiety is one of those things that can sneak-up on a person, without you being aware of it.
i myself do suffer from asthma problems since i was 6.
i do now and again feel faint & dizzy in the shower.
i think that you need to get this checked out by your doctor. it might be something unrelated to asthma but the best possible way to find ths out for sure is to see your local GP.
i hope this has helped?.