I nearly forgot about the magnesium. Good reminder. My nephrologist told me I shouldn't use magnesium or potassium either one until he gives me the green light. So, naturally, I forgot about the magnesium, which is a wonderful option. Most people are actually deficient in magnesium anyway.
Take Magnesium Citrate 250-1000mg to start with each meal and it will cure your constipation. Start with 500mg every meal if still constipated then up the dose, but if you get stomach upset then take less.
Did your doctor run you through a series of tests to confirm this is IBS and not something else, such as the colonoscopy? If not, you should insist on it.
Have you looked into food sensitivities? Something similar used to happen to me when I first learned that I had IBS. It wasn't until years later that anyone told me about the connection to food sensitivities.
Try researching information about the food elimination challenge diet. There are about 16 different items on the list of foods that one must avoid eating for an initial period of time. Then, the foods are introduced one at a time waiting for symptoms to reappear. If you get symptoms with the suspect food, then you will need to avoid eating that food item again.
Try adding flaxseed meal into your diet instead of psyllium fiber. Psyllium fiber made me bloat like a balloon and irritated my GERD and never really relieved the constipation. I'm sure you will get a measure of relief when you use the flaxseed meal versus the psyllium fiber.
I have noticed that conventional doctors don't give patients enough information about diet that could save people from having to suffer their symptoms so much.
I urge you to find a naturopath to work with you while you go through the elimination diet. A naturopath can help you with your symptoms if the elimination diet is still not enough. My naturopath has me taking a couple of herbs known for helping with constipation. They're not supposed to contribute to my kidney issues either. But, it's really important to see how much relief you can get through dietary changes first of all, before you go to using any of the herbs. My naturopath told me the same thing.
It takes quite a while to go through the elimination diet. So, you will need patience and endurance to get through this elimination diet. I think you'll find it worth your while, though.
You must only use the herb as a last resort, because you need to see what your body will do on its own without any laxatives.
I think you'll find that the two biggest culprits for making the constipation so much worse are dairy and gluten. I don't mean just wheat, although that the biggest one. Gluten containing grains to avoid are: wheat, spelt, kamut, triticale, rye, barley and oats. You'll need to avoid corn, too, eventhough it doesn't contain gluten, because corn is highly allergenic and is on the list of foods to challenge on the elimination diet anyway.
Also, no soda. Soda contains high fructose corn syrup, which is the worst kind of "sugar" there is. You can find such information about corn syrup on mercola.com. Not only that, it's still made from corn. Everything you drink that isn't soda, be sure to read the labels. And, actually, you need to avoid all obvious forms of sugar, which means honey, maple syrup and agave nectar and anything else I might have left out. Water is your best alternative. Read mercola.com for the best guidance about which green teas he recommends. It's really important to get the right kind, because too many of them actually contain high levels of mercury. Don't forget that black tea, green tea and white tea all come from the same plant. Tea is also a mild diuretic.
Also, avoid artificial sweeteners. Don't trust any of the chemical alternatives to sugar. There is no panacea for artificial sweeteners. They're all bad.
Definitely avoid coffee. It's highly allergenic and it's a strong diuretic, which you do not need with IBS. Coffee not only irritates my colon but it adversely affects my asthma and makes it difficult for me to breathe.
I hope this information helps you. I'm sure it will make a difference while you are waiting for your next doctor appointment.