Why does bronchitis make my blood pressure swing so much?

Cough or meds: Increases in blood pressure during infection are usually secondary to coughing (which can raise intravascular pressure) or from medications (mainly decongestants). These are the two reasons your blood pressure may be elevated during an active episode of bronchitis.

Work of breathing: With bronchitis, sometimes breathing is harder, so one has to generate more negative pressures in the chest to take breaths in and more positive chest pressures to breathe out. A more negative chest pressure will make blood pool in the chest longer, making your blood pressure lower temporarily, and the more positive chest pressure will push blood out of the chest, making your blood pressure higher.