when to go to ER for chest pain?

Sarah

Okay so about an hour ago (?) I started getting this pain in my chest on the left side. I'm 18 and I've been diagnosed with tachycardia, as my resting heart rate is ALWAYS 90-120. I'm pretty sure this is related, but I'm not sure when or if I should go to the emergency room for this? I'm not short of breath or lightheaded. I have a slight headache but that's normal for me. Any opinions? I'm pretty scared tbh, I dont know what to do...