Hello! I too have experienced similar stomach problems, my stomach would get bloated and really hard for no reason, people would ask if i was pregnant as im fairly tiny but had this huge pot belly. My doctor thought i might have a wheat intolerance but turned out not to have. I also get extremely bad stomach aches and feel very very sick for no reason. I think this is a very common effect of panic and anxiety. Have you talked to your doctor about it because I'm sure there is a lot you can do to combat it.
With regard to your flight have you tried Bachs rescue remedy? You spray it onto your tongue to instantly calm you, its totally natural made from flowers. Im not sure if its psychological but it really calms me down. you can do some great courses for fear of flying i think Virgin has one. Also do you do a relaxation or breathing cd or tape at all? If you learn some simple breathing exercises to calm and distract you then you can use them in situations where you start to panic and concentrating on them really helps take your mind of things. I would also suggest taking a walkman or ipod and tons of books, puzzles and magazines as keeping busy stops your mind racing and panic starting. Maybe you could tell a stewardess you are a nervous flier, last time I flew I was in tears with the fear and the stewardess really took care of me. Sitting in an aisle seat helps as you feel you can escape if you need to and your not gazing out of the window. you should give yourself a huge pat on the back for getting on that plane, I havent flown for 6 years due to my anxiety about it so youve made the most important step by booking that ticket and actually being brave enough to conquer your fears. Good luck let me know how you do.