i had a yorkie that would seem to have breathing problems when she got too excited (like when i came home after being out) - it wasn't a collapsed trachea. as soon as i would pick her up and pet her she would calm down and be fine.
here's an article on collapsed trachea, it does mention inflammation could be a problem which benadryl would help, but i would not self medicate a problem especially with no firm diagnosis. i would get an air purifier, stop smoking (if you do), avoid getting her hot or putting her in situations where she pants, not let her get overweight, etc. first.
http://www.marvistavet.com/html/tracheal_collapse.html