Hi Daniel,
"My domain is only 4/5 months old but was already becoming relatively competitive"
If you get ranked very quickly upon you released the site it might be the case that Google ranked your site much better than it should be because that's what Google do. Some people say it's the best to do backlinking for the first seven days upon launch of the site and before the Google index it for the first time.
Newly sites are usually ranked better in order to determine their real value so they drop a couple of months later unless you apply good SEO techniques to keep it there. When you say suddenly, how long did you mean? Few days, a month or it was getting worse and worse for a few months?
If you type site: www.danielalexandra.com/personal-training "personal training" you will see that the first page in the eyes of Google is http://danielalexandra.com/personal-training/personal-training-packages/ for "personal training" keyword. Is this the keyword you are trying to rank for?
Personally, I would pretend that nothing happened and continue building quality backlinks to support the keyword.
Good luck!
