Thats not very long and I assume you have never met her. You seem to be an all right chap. Please be aware that some countries are well known for loverboy/lovergirl scam methods.
I strongly advise you to send money only to people you have met in real life and with whom you have established a real connection.
Never send all your money and never borrow money for this type of action. I am not saying its your case, I really wish that the connection between you two is real
Ok Justin. Money orders seem to be a thing of the past as it is a very inconvenient way to send money. I found this explanation of why your gf cannot cash in on trip advisor though the post is 12 years old. Also I'd suggest she returns the money asap so you can try a different method.
I STRONGLY ADVISE YOU TO CALL THE COMPANY AND TRACE YOUR MONEY.
There is also the posibility the money was cashed out but there are no means to verify this.
Off-topic : May I ask how long do you know your gf?