If this started suddenly, I would be concerned about the possiblity of a incarcerated hernia - a hernia which has a piece of intestine or tissue from the abdomen trapped in the sac.
This is a urgent surgical issue - if the hernia sac can not be reduced, you may need to have urgent surgery, because there is a risk that the intestine could have it's blood supply cut off - leading to a serious medical complication.
I would suggest a visit to the emergency department tonight.
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