Unlike iOS, where Instagram is shown as an icon next to Facebook, Twitter, and LinkedIn, there is a different sharing flow for Instagram on Android.
1. Tap the 'Share' icon on the post you'd like to share to Instagram:
2. Tap the icon with 3 dots, then tap 'Share':
3. Tap 'Instagram Feed' to share to your Instagram:
4. You can resize the photo, then tap the right arrow:
5. You can choose a filter, then tap 'Next'
6. Write a caption, then tap 'Share'