Re: What causes rapid weight loss...
Rapid weight loss can be caused by cancer. It can be caused by other things, but don't wait on this. Force the issue with your doctor. The cancer mechanism is, as tissue cancer functions inefficiently and uses up your body's glucose, causing weight loss.
As for the lower left abdomen pain, that can be diverticulosis - the mild form of diverticulitis. That can't really be treated, you just need to avoid hard dense foods, like nuts, seeds, and popcorn.
The weight loss is very concerning. Talk to your doctor, tomorrow. Don't take "it's nothing" for an answer.