When the body's core temperature goes up, or "we have a fever", the body's natural response is to cool things off. This is done by the natural dilation of the blood vessels in the body, in an attempt for them to cool faster (think ice water bath for a high fever). When the blood vessels in the healthy circulatory system dilate, blood doesn't have to work very hard to circulate blood, so when the vessels enlarge, the blood pressure goes down. Fluid follows the path of least resistance, so if there is no resistance, this also helps the blood pressure go down. Thanks for a great question.
Ladywitch
Ladywitch