Could my diet be affecting my tendonitis?

Yes: You ask a very interesting question and I am sure somewhat controversial for some. The most common reason for tendonitis is overuse due to the repetitive microtrauma. Eating the standard american diet of sugar, saturated fats and bad carbohydrates will cause inflammation in your system that effects everything. So yes, consider eating a low inflammatory diet (google it). You will feel bettter.

Tendon diet: A salty diet or one that is associated with eater retention nosy in some cases exacerbate tendinitis.