I have practiced anapanasati for a while and encountered some difficulties. I can easily find my breath in front of nostril. Then I started to pay attention to the in-and-out of the breath continuously. But after a while, pain raised in my chest around heart. As I tried to focus more on the breath, the pain increased. In addition, if I suddenly stopped paying attention to the breath, I found my neck and shoulder very rigid and uncomfortable. I also felt exhausted after the meditation.  

I’m wondering if I focus too much. Before I know anapanasati, I used to practice simple meditation like just relaxing and checking any thoughts if they arise. I felt much better by not paying attention to any object.

 So my question is: what is a good “attention” to breath? Thanks!

Mingyur Rinpoche teachings can be helpful for you, I think. For example here - https://learning.tergar.org/course_library/intro-to-meditation/session-2/

The first month of my practice all progress was accompanied by rising muscle tension which on occasion resulted in pain if not recognized early enough (it's difficult to notice involuntary muscle tension, after all, even outside meditation).

The whole month the entire game was concentrating, checking what tensed up, releasing the tention and then go back to concentration.

Weirdly, at the end of the month there was some sort of tension-climax after which they were totally gone and never returned substantially in subsequent practice.

Maybe you are experiencing something similar?

Read the progress of insight stages at MCTB.org