I had a very bad headache on left side and my throat hurts this started 4 days ago 2 days after my headache my left eye became blood shot. What could have caused this

You could have a blood clot that moved up your face, caused the headaches, and then caused a vessel to break in your eye. Or an infection in a tooth or your throat, also spreading and hurting because of swelling... there are any number of possibilities. Of course you need to be seen by a doctor.