I was asked this question this week.
Often I see people with upper respiratory infections who are very concerned that when they blow their noses they see blood. They get very worried that this means they are terribly ill or that they definitely need an antibiotic.
When you have nasal congestion the inside of your nose can get very dry. There are a lot of small blood vessels inside your nose. When you blow your nose, they can break. This is nothing to worry about.
Now you know.