Increased heart rate can be a sign of feeling anxious about something. When this happens take some deep breaths (blowing on your thumb so you can feel the breaths can help) and this should help your body to calm down.
Your heart is beating faster because your body is preparing for fight or flight responses which actually can make you react better to stressful situations as your body is prepared for stress- unsure of the exact science but watched something on it at school.
If you’re finding it hard to deal with being anxious or stressed then maybe use a site like Kooth to find resources to help you with this.