Poop!!!
Frequent loose BM's are a hallmark of the DS, and one reason why many are so put off by it. It takes about a year for your body to fully regulate. First year for me was murder, after 6+ years one hour, three times in the AM and I'm done for the day, unless some food or drink irritates my system.
Become a label reader. Lots of hidden stuff in processed foods can set off negative effects, as well as bind you up. Simple carbs and simple sugars are the worst culprits for dhr, overdoing it on fiber is worst culprit for constipation. Find your normal, and stick to what works for you, using generalities as a guideline only.
It may be that the drugs given during surgery has caused this problem. Eventually that wont be a problem and for awhile it will be opposite. You will be dropping a BM like a Lion in no time!!! At least that how most DS'ers turn out... BTW Miralax is probably the best bet but ask the Doctor or staff what they recommend.