1-Skin rash or burn 
Prolonged use of hot packs and heating pads, or application of a heat source that is overly hot without a barrier on the skin, can cause contact burns.Due to the sudden decrease in blood pressure, individuals who often experience orthostatic hypotension (dizziness or light-headedness upon standing) may find certain heat treatments problematic.3-Increased swelling and inflammation
Heat therapy is not recommended immediately after physical activity, after injury, or during an infection.