Is it possible for shoulder fluid to get infected?

Yes: It is possible but typically rare. However, if there is any concern for an infection (redness, warmth, fevers, chills, drainage, swelling), you should seek immediate medical evaluation.

Shoulder infection: It is possible for any joint to become infected. Septic arthritis can occur from hematogenous(blood stream) or lymphatic spread or from penetrating injuries or injections.