What can cause fever blisters without having a fever?

Cold Sores? Triggers for cold sores can include illness, stress, fatigue, becoming physically cold, dry air, UV rays (sun), mouth trauma or one's period.
Herpes, shingles... Several things can cause blisters: viral infections (herpes, chickenpox / shingles [varicella], for example), adverse drug effects, insect bites. Talk to your doctor if you are experiencing this condition. .