I have read that 95% of spotting cases turn out to be uterus cancer and I am extremely worried!!!!!
Where in the world did you read that?? That number isn't accurate at all. If anything, it's the other way around.. ESPECIALLY in a 20 year old...
Women have cycle irregularities quite often. Hormones can fluctuate. You have been losing weight and started a new exercise program this is enough to cause your cycle to change. The weight loss and exercise have cause your body to change the way it regulates hormones, causing the spotting and wacky period.
Stop reading about uterine cancer on Google. That will serve no purpose, other than to further your anxiety.
Even if you called a GYN and asked a nurse...she'd tell you that they don't even begin to investigate spotting until it's gone on for more than 3 cycles in a row...and even then (in a 20 year old)...there are a million other causes, besides cancer.
I've had GYN issues since I was a teenager. I've had every symptom you could imagine. I don't have cancer and neither do you.
Focus your energy on being healthy. Mind and Body. You are going to be fine, I promise :)