Belly automatically tense whenever focusing on breath or trying to relax

Every. Time.

The moment I focus on my breath, I am no longer able to "breath naturally".
I only breath naturally when I am eating, reading, on the computer, etc. If in the middle of these tasks, I notice the breath, I see it is naturaly and relaxed. But if I keep watching it too long, it starts to tense up.

Has this happened to anyone else?
If so, have you found a way to relax?

Probably counter-productive, but: Is there any breathing pattern you've found beneficial for concentration? In other words, a set timing or form of breathing that leads to a relaxed, effortless belly?