What Causes Blood Blisters In Throat After Undergoing Tonsillectomy?

Hi,Today is my 4th day since my tonsillectomy and after I spat out a slight amount of blood I had a look at the back of my throat - there seems to be a blister filled with blood where the tonsil used to be. I can feel it with the back of my tongue and after I gargled it didn t come away or start bleeding any more/less. What is it?!Thanks