Lie on a bed on your stomach firstly with your head turned to the left then with your head turned to the right. See if you notice any difference in particular, if there is a side which feels more comfortable/natural or whether it feels exactly the same regardless of which side your head is resting.
The purpose of this is to determine whether there is any difference in the flexibility with regard to the rotation of your head.
Now, if it feels more comfortable or natural lying with your head turned to the left and awkward with your head turned to the right, then it's more likely than not you have less flexibility on the right.
How does this translate to your breathing and your stroke? Well to put it simply, your head is turning less on the right so your mouth can't clear the waterline to take a breath. The natural instinct will be to lift your head which will then cause your body to dip resulting in a loss of balance and stability. You will then lose your streamlining, and will need to start your left catch earlier to compensate.