I am 5 weeks post sinus surgery, today i woke up with a swollen face and fever blisters on my lower lip. what could be causing this?

Unrelated virus: Viral infection unrelated to your sinus surgery. Contact your primary care doctor for an evaluation.

Trauma reaction: Herpetic blistering can often come about as a result of some trauma, nervous tension, sun or wind exposure etc... Sinus surgery certainly fits the bill as to trauma, hsv type 1 virus is known to live in the nerve trunks of the face, and now the surgery has acted as a trigger for one of a lifetime of breakouts of viral blistering (cold sores).