What Causes Blood In Saliva While Suffering From Tonsillitis?

Hi ive got severe tonsillitus but when ive been spitting out saliva, beacuse im struggling to swallow, ive noticed blood in my saliva. It only tends to be at night too. The first time i noticed it was when i was sick on sunday which was the first day of me having it. My doctor has prescribed me penicillin if that help in anyway. Thanks