What Causes Weakness, Nausea And Headache After Taking Vaccination For Yellow Fever?

Hello there. I had a yellow fever vaccine 5 days ago and have been fine since - until today. I feel very weak, slightly nauseous and have a headache (been in bed all day!). Could this be a reaction at this stage or is the time lag too much to be related to the vaccine? Thank you. Mark (32 years old, 6' 2'', 11 stone and otherwise healthy)