Not to worry Sara, it happens to everyone for various reasons. Including me.

Your hands, wrists and fingers will eventually become stronger the more you practice.

You say you experience strain trying to reach those two notes, every guitarist is different to the next, meaning what instrument works for them might not work for the next person.

It's recommended to try the guitar you're interested in before you buy, I remember a scene from the first Harry Potter movie. Harry enters a wand shop and the owner helps Harry find the right wand for him. Some are the wrong size or shape, others are too powerful, some do not work at all. Harry eventually found the right device for him.

It's the same when choosing a guitar.

Here's the current thread on the subject:

http://www.guitartricks.com/forum/showthread.php?t=35522

Hope that helps ;)