I got those quite often when all my anxiety issues started - they'd occur in different parts of my chest and sometimes would shoot through to my back. They still surface occasionally, but it's pretty infrequent now. My doctor said they were probably one of two things (or a combo of both): muscle cramps from tense chest muscles, or nerve firings due to tense muscles putting pressure on the nerve endings. I found that if I was able to relax my chest muscles when they started (difficult, of course, because the immediate response is to panic), they usually went away much more quickly! Tight muscles can be really painful, even if you don't realize you're tense.