Sigmoid colon cancer removed surgically. After 10 days ultrasound impression is ' subacute small bowel intestinal obstruction and mild intraperitoneal free fluid collection. Is this common?

No, not common. Bowel obstruction can occur due to post op adhesions(uncommon) or tumor left behind(this will happen if the cancer was advanced/late stage. Your doctor will have to sort out which of these two possibilities apply to your case.
How are you feeling? You had a sigmoid colon resection without colostomy. You are having an ultrasound to rule out anastomotic breakdown or fluid collection. But how are you feeling? The ultrasound does not tell much about obstruction but is good for fluid. If you feel ok and are walking around a lot then then bowel function will return. Try chewing gum of you can't eat yet. Hope you get well soon.
Yes and no. While a small amount of intra peritoneal fluid is common after a colon resection, a small bowel obstruction is not common 10 days after surgery. Relief may require small bowel decompression. Increased activity and diminished narcotics use may also help to stimulate small bowel function.