I have been working on some relatively complex web app and since the beginning every two weeks or so my boss decides that we should bring some relatively important changes on the flow of the app on screens that i already made. Anyone can relate to this? Is this common in the tech industry as a front end dev?

This will lead to scope creep, late deliverables and worst of all unhappy internal and/or external clients 🙁
The requirement must be documented and signed off on before development. If there are significant changes they can be incorporated into version 2. If there is a critical need to add them to V1 that is fine - there are exceptions - but usually they can go to V2. The will allow for on-time project delivery and happy clients 😀

@Jumpshot44 yeah, or exactly the opposite of that :/ if the requirements have changed, why continue to program something nobody wants?
You're describing waterfall methodology, which has been terrible for the last few decades. Modern methodologies use Agile, which account for changing requirements (like, you know, in the reality)