I had mild asthma as a child, don't suffer now, and haven't for years.
Since you are only 20 there is a very good chance that your very mild asthma in childhood, and what was done about it, will be well documented in the records at your GP's surgery. If that is the case, then asking your GP to provide a summary ( a paragraph or two would be plenty ) for you to take with you when you go could work wonders.
You would also be well regarded for being so thoughtful and well-prepared !
Good luck with the medical !