It is only natural to be concerned about having to explain yourself to new people, but remember that you do not owe them an explanation of your condition.
Have a short explanation ready
Sometimes, preparing a short and confident explanation will help put your mind at ease for the rest of the evening. For instance, you could explain that you've been unwell, and because of that you've had major surgery and now wear an ostomy pouch.
Avoid being too technical, and be open to questions - without over-sharing. Never feel you have to explain details of your surgery to everyone who asks.