Blood from the umbilicus could point to several reasons. It could be from a mere bacterial infection to a malignancy/cancer. Considering your age, it is unlikely that it is cancer. Make you sure you didn't scratch it during accident or have a piercing in the area.
Without directly seeing the scenario, it is hard to tell.
Next Steps
You might want to try a course of antibiotics as directed by a registered medical practitioner. You could also do an Ultra Sound imaging, in case you're very worried about an growth inside your stomach.