HELP: IMages automatically shrink when I insert them

Suddenly this morning there is an issue with uploading and inserting images on my blog. When I insert the image, it is immediately shrunk down to a smaller size, not a thumbnail, but just a smaller ratio, and for no reason I can think of. I can’t just go into the html to change the size automatically because then it stretches the image and distorts it. Basically I upload an image that is 550 px wide and suddenly it’s 300 px wide when I put it into the post.

Any ideas?