First understand I am not a Dr.
I do wonder how old you are & whether you might be going through teen years body shape/hormonal changes which might be at least part of the problem?
I'm sure (my son had childhood asthma) that there are many different types of asthma medications & perhaps you should talk to your dr about trying a different one? There are I have been told also some apparently excellent alternative (natural) remedies available for asthma too??).
As to your tummy - do you do any sports? Has the change in your asthma situation caused you to give up any sports? Stopping could be the problem & also not doing - so if either applies perhaps there's some sport or even exercises (yoga/pilates?) that might help tone your tummy muscles.
Good luck