My 11 year old daughter has an irrational fear of wetting her pants from laughing too hard. It has apparently happened where this occurred in very small amounts, but now she has turned it into something that is holding her back from doing things. She is so afraid that it will happen and people will notice. she can't sleep from worrying about it.
Irrational fears are hard to deal with. However, this sounds more like she has already experienced this embarrassing event, and now this fear of a recurrance has become overblown in her mind. This is a bit easier to deal with than an irrational fear.
Assure her that this kind of accident happens to every female at some time or other. It is nothing to be ashamed of, although obviously avoiding it is preferable. In young women/girls, it usually only happens because they have a full bladder and have ignored emptying it (after all, there are more interesting things to do with one's time. :)) It might to help to explain (maybe with a diagram?) that if she has a very full bladder and something suddenly causes a sharp increase in intra-abdominal pressure, like hard laughter, the pressure can push a bit of urine out of her bladder.
Reinforce to her that if she goes to the bathroom as soon as convenient after first feeling the urge (or better yet - for now - at regular intervals, even before she feels the urge), it is likely not happen again for many decades.
It might also help her confidence initially to wear an ultra-thin absorbancy pad on certain occasions when she is out with others. Showing her that fluid will not pass through the plastic barrier (therefore will not show on her clothes) may help to reassure her until she learns that frequent bladder emptying is sufficient to preclude this from happening, which should not take too long.
If all this fails, I would bring it up with her pediatrician. Sometimes a doctor visit, a physical exam, and a urinalysis might provide the reassurance she needs (and might also detect a problem if there really is one that needs to be addressed.)
I would caution that the pad needs to be clean, and if she does happen to have an accident, must be removed within a certain amount of time to prevent a possible bladder infection.