What Causes Heart Palpitations, Dizziness With Syncope Sensations?

My daughter is 15, 5 10 , 130lbs. For about 5 days now she has been having episodes where her heart pounds very hard and fast, at rates up to 130 for periods as long as 45 minutes. When this is happening she is usually at rest in her bed or sitting with no activity. If she stands it does increase as much as 20 beats. She also doesn t feel good prior to and after. When I ve asked her to explain how she feels, she just says she doesn t feel good. Also, she has for the last few years experienced dizziness to almost the point of fainting upon standing,, where she cant see. This can happen on a daily basis. We have seen the Dr. about this and they just dismiss it as she needs to drink more. She was seen 2 nights ago at a childrens ER, where they did an EKG, saw nothing and sent us on our way to follow up with a cardiologist. Im just wondering if I should be worried or not?