I have IBS too and take peppermint oil tablets. I also take mebeverine which is an anti spasmodic tablet which helps with the tightness ect, might be worth asking your gp for some, it will help.
How long have you been taking these tablets for because if you're only going once every couple of days this might just be the tablets clearing your system out after a long time of constipation. You should go at least twice a day. Unfortunately I have the opposite problem, I go up to 20 times in a day mainly before midday. I wake up in the morning with severe cramps and painful wind, then I'm on and off the loo all morning, sometimes all day. I also get lumps in my abdomen which my gp tells me is solid food in my bowel. This can cause your stools to be long and thin as it's having to squeeze past the semi-blockage.
IBS is something the gp's don't really know anything about. It's an umbrella term for "something wrong with the digestive system". The drugs they give you are all trial and error so if you really aren't getting on with the tablets you're on then go back and see your GP, there are loads of different tablets for IBS depending on your symptoms.
Hope I've shed some light for you and I hope you feel better soon