I just threw up what i think was blood and now my chest hurts. what cold this be and what should i do?

Seek medical care: Vomiting blood is a red flag and deserves immediate evaluation especially with pain afterward. You could have an ulcer or a tear in the esophagus.

Chest Pain: See a physician as soon as possible it may be nothing, but forceful vomiting can cause a tear in the esophagus (boerhaave's syndrom).

Vomiting: It is common after vomiting to have some heart burn like symptoms. But if you have chest pain, it needs to be evaluated. Go to your doctor asap.