Instagram switches to a feed based on an algorithm, to make sure you see more when using the service. From their blog post, which doesn’t contain a lot:
The order of photos and videos in your feed will be based on the likelihood you’ll be interested in the content, your relationship with the person posting and the timeliness of the post. As we begin, we’re focusing on optimizing the order — all the posts will still be there, just in a different order.
Prepare for a public outcry, folks.