Images Suddenly Appear Thumbnail Size

I haven’t changed anything on my site in a while except for updating WordPress and plugins, and today I noticed that my images are appearing thumbnail size instead of full size. I’ve gone in and deleted the image, re-uploaded it, formatted it, and updated the page…but when I go to the site it’s re-sized to 150×150 automatically. I went back to check the code and it still says it should be full size, but it still appears to be automatically shrunk. Any ideas?