A while back, Twitter announced plans to optimize people's feeds and show them the "best Tweets first". Based on how a user engages with certain Tweets and other users, as well as who they follow and what Twitter deems their interests to be, they'll be shown new tweets according to a ranking system rather than chronologically. While this means that you'll be seeing the content you care about the most if Twitter gets the details about your usage on the money, you could also end up missing Tweets from friends, favorite celebs or companies. If you'd like to go back to the chronological feed, however, you're in luck; Twitter has left that as an option, both in the app and on the web.
To switch back to the old way of viewing your Twitter feed in the Android app, you'll need to give the three dots in the top right a poke, or hit the menu button on your device, if you have a device that sports one or a custom ROM that allows you to stick one in your navigation bar. From there, click on "Settings" in the menu that pops up. In the settings menu, you'll find an option that says "Timeline". Tap that and you'll be whisked away to a rather sparse menu that has "Show me the best Tweets first" as the only option. Uncheck that and you're back to the way your Twitter feed was, viewing Tweets as they come down the pipeline.
On the desktop website, simply log in and head over to your settings by clicking your own avatar near the top right and clicking on "Settings" in the drop-down menu. You'll see multiple headers as you scroll down, one of which should be "Content". Under content, you should find Country, Tweet media, Video Tweets, then Timeline. Under that sub-heading, there should be a checkbox, identical to the one on the mobile app, that says "Show me the best Tweets first". Click that to uncheck it and your feed will be back to the way it was before the change rolled out.