Can an injection into my knee cause high blood pressure? the injection was five days ago...bp soared to 236/116 four days after the shot.

Possibly: Pain is a more likely reason blood pressure goes up. However certain types of local injection can be absorbed systemically and lead to untoward side effects. Injection steroids for knee pain, can lead to fluid retention and hypertension in some individuals.

Unlikely but..: Make sure your BP machine is working ok. Unlikely that a cortisone shot into the joint would increased BP but pain might increase bp, also if cortisone shot was into the tissues instead of joint it is possible that it would have a small effect on bp. But it sounds most likely that 5 days later, an elevation in your BP is most likely due to something else.