it's not really possible to lose much weight at all in such a short time period (of real not-just-water, keep-it-off weight, you'd be looking at 1-2 lbs, tops). BUT you can do a few things to make yourself look trimmer, i.e. to look less bloated/retain less water.
here are some tips I've learned over the last few years from friends who do bodybuilding competitions.
this is what they do in the week or so before a competition to get their bodies looking as "cut" (slim, so that the muscles show through) as possible. these are pretty extreme but whatever you can apply to your own life along these principles will help!
1. stop eating salt completely. and read labels - salt can show up in some weird places. don't eat anything that has any salt as an ingredient.
2. stop eating sugar - the obvious things like sweets and cookies and candies and sweetened drinks, but also again, read lables. sugar even more than salt gets added to weird things. (watch for things like soy milk and seemingly savoury bottled sauces, like ketchup or stir-fry sauces etc. even the littlest bit of ketchup counts when you're trying to lose just the littlest bit of "weight".) also stop drinking any fruit juice or eating any dried fruits for the time being.
3. cut back on starchy carbs as much as you possibly can. stay away from breads and pastas and cold cereals especially, and try to get most of your SF from veggies like squashes, potatoes and sweet potatoes and SF supplements. have some oatmeal and/or rice (if you can tolerate brown rice, even better, white basmati rice would be ok too) but not too much.
4. you probably already do this for your IBS, but don't drink any alcohol at all. alcohol is like a super-sugar.
5. drink water like a fiend! as much as you posibly can. your body retains water in its tissues so it can use it for later. this (water retention) can give your skin a slighly bloated look. but if you give your body enough water when it needs it all day long, it won't need to store some for later = less water retention = less bloating. (remember though that female hormones can also cause water rentention - so depending where you are in your menstural cycle or what your body's hormones are up to lately, fighting water rentention can be easier or more difficult.)
6. eat less, but eat more often. have more smaller meals - like 5 or 6 instead of 3, spaced out evenly throughout the day - and eat a bit less (not hugely less, but just a bit less) total food all day. (if you count calories, cut by about 500 calories total each day.)
7. and lastly but importantly, yes exercise! cardio is what you want right now. depending on how good shape you're in, go for a run or walk or jog or bike ride or whatever for 30-45 minutes each day, and if possible twice a day is best. just do more than what you're doing now - and if that's nothing, than whatever you do will help. just remember, the more intense the better.
again, just do what you can!
hope that helps!
happy (early) birthday!! hope you have a great time - and good luck!