Sometimes, including three more months makes your working period seem even shorter, and over-explaining attracts more attention to your awkward situation.
If you feel obligate to point out, use as few words as possible.
The title is something your company gives to you, right?
So whatever you can use, as long as your company confirms. ^_^
Besides, I think people are more interested in "in charge of a new project in the South China market, leading a team of 4", rather than what your position description is.
Also, perhaps it's not a bad idea to tell them that you started with a Sales Representative and soon became a Project Manager, and why so.
I mean, my title was a sales rep, but I performed as a project manager in my daily work. I did not become, but was at the very beginning.
My company printed the latter title in the official business card but they were not willing to change it in the contract.
You know, some companies would like to print a much more decent title in business card to their employees who need to meet clients and negotiate projects.
maybe I should choose the former one in case of background check. Confused!...
However, if I were you, I would begin one of my essay with "Although my official title is a Sales-Rep, I have no idea or have done anything close to what the title describes. The story should begin at my first interview..."
I think it will be fun, but of course, this is my style.
Anyway, I don't think it is anything you should worry about. Just make sure your story is true and what you says seems true.