Try taking either peppermint supplements or aloe vera juice (just a teaspoon or so a day), it should ease the cramps and needing to go too often. If you take too much aloe vera though it will clog you up from here to next Wednesday.
There arn't an awful lot of drugs out on the market that seem to effectively ease IBS symptoms, but it can be triggered by certain types of food, so you might want to have a look at that too.