Welcome to the gastroenterology community! People can indeed have IBS. Please do not doubt your diagnosis. IBS cannot lead to chrons (I checked with a doctor about this). Have you had an IBS to confirm that you don't have something more serious. If this is the case, you do not have inflamed bowels. Miralax is perfectly okay to take if trying increased fiber, fruits, vegetables, water, and exercise didn't help. However, to answer you question, I would say that you shouldn't be concerned. Many people with IBS have strange bowel movements (size, consistency, frequency, etc.). You should check with your doctor about this though.
No one should have IBS. Your bowels are inflammed due to an chemical imbalance. You need to go back to the basic diet with no chemicals added. This means Fresh fruit and vegetables, plenty of spring water, no red meats for now. No breads...(breads have yeast which wreaks havo on your systm, also has mycotoxins....both of which is not good for your body at all. No bottled drinks except water. Fruit juice should be 100% frozen...no processed. IBS can lead to Crohns and you don't want that.
Go to health store and buy pH test strips and see how acidic your body is. It should read 7.3. ..anything less is too acidic. Wheat Grass will alkalize your system. Aloe will calm the inflammation. Probiotics (buy at healthstore) and take food-based vitamins/minerals. Buy psyllium husk for fiber..Miralax has chemicals in it. Take care of yourself and you will no longer suffer from this!