Odd things can happen when you start to get concentrated. Particularly if you "try" too hard. I've heard of people getting headaches because they were focussing on the breath at the nostrils and went cross eyed inadvertently trying to "see it"...
You might find Ajahn Brahm's introductory instructions useful, even if you are not using quite his technique.
You can download the first few chapters of his book, Mindfulness, Bliss, and Beyond here: http://ebookbrowse.com/ajahn-brahm-mind ... -d84597124
Take a look at chapter 1 and see if any of the advice there helps your problem.
An approach I find is useful if I find myself "forcing" things is to just be aware of the body, and wait for the breath to become obvious, rather than "going after" it. This can give a more relaxed approach.