In an interview on The Cultured Nerd with Marvel scooper Roger Wardell, he explains how Ant Man will tie into Age of Ultron:
Ant Man becomes about some of the stuff leading up to and right at the same time as Age of Ultron. The timeloop might have happened many times, but what we see in Age of Ultron and Ant Man is its last cycle. Some parts of Ant-Man will take place DURING Age of Ultron.
For a long time leading up to Ant Man being official, Wright described it as a standalone movie. Obviously that’s the exact opposite of what Marvel is doing with their shared cinematic universe, and Wardell went on Twitter to explain that these changes to the Ant Man script is why Wright left: