Pain in sides with exercise/My sweat response is weird

When I exercise, I get this almost unbearable side stich usually on the right side, sometimes left, sometimes both sides-the pain is so intense that I can barely take a full breath in.  It isn't only with intense exercise-I walked around a moderate sized track today, and on the first lap, not even half way through the lap, the pain was on both sides and I couldn't stand straight, or expand the abdomen enough to breathe in all the way.  Do you have any idea what this could be?
I think my sweat response may be abnormal.  Sometimes I won't sweat at all during a workout, or I will sweat a little, but usually I sweat profusely after the workout is finished, sweat dripping everywhere. My face, arms, and chest also become very red, and sometimes I will be so hot (sweating very little or not at all) that I feel cold.  What is a normal sweat response?
Thanks.

[sloopy]

By the way, I am 25, weigh 99, and am 5'6.-amanda

[Tyler King]

If your working very hard and not sweating you might be dehydrated, make sure you drink plenty of pure water every day and double your intake when working out.

[hbp702]

99 pounds for a 5 foot 6 woman is a BMI of 15.98.

You need to see a doctor ASAP, that is way low, and indicates something very wrong, such as anorexia nervosa or some other eating disorder or some other cause of poor appetite or malabsorption, hyperthyroid or something.

115 pounds is about the minimum safe level (BMI = 18.5) which would  still be quite low.