Sudden pain is always of concern, especially if it worsens or persists or is accompanied by other concerning symptoms, like fever or nausea and vomiting
. I recommend talking to your primary care doctor
as soon as possible.
One concerning possibility would be kidney stones
. These manifest as severe pain in the flank which moves into the abdomen and down the leg, sometimes to the groin. Classic of kidney stone pain is that it is crampy in nature and comes and goes, with periods of severe pain alternating with episodes without pain. I am particularly concerned about this possibility, because kidney stones sometimes occur more frequently in people who are experiencing rapid weight loss or dieting.
Another would be a kidney infection, also known as pyelonephritis. This usually begins as a urinary tract infection with the typical symptoms of burning with urination and then progresses. Kidney infections have severe pain in the abdomen or in the lower back just under the ribs. Often there is fever as well.
It sounds like you should call your primary care doctor as soon as possible to help you figure out what is going on. If fever or severe pain returns, you should go instead to the emergency room.