I have only recently started doing bikram (5 classes so far, loving it), and whilst i have no health issues, i have similar sensations when going into camel pose, it has almost been to much to just lean backwards with my hands on my hips – i feel as though a panic attack is beginning. My hypothesis is that (for me) it is more emotiona-healthl than physical-health related. I feel it is symbolic of opening up my heart or something else related to revealing myself fully. I react really strongly to this pose, but have no problems with (attempting) any of the others.
Having said that, its always a good idea to get a health check-up just in case, better safe than sorry =)