I am a teen girl from the UK. I have had a lot of bloodtests done as I have been feeling very tired and dizzy and near fainting. The results have all come back normal apart from a calcium deficiency which I am now taking tablets for. 

As well as the tiredness and dizzy spells, I feel sick often and feel like my heart is pounding and beating too fast and sometimes I also feel like my heart is skipping beats. I dont know if it actually is doing these things but thats what it feels like. I also have some chest heaviness and shortness of breath, I have asthma but it does not feel the same as when my asthma is bad. 

I have not yet had any testing done on my heart so Im going to ask my doctor about this, I would like to know what sort of tests I should ask for as I am worried I have an irregular heartbeat. I read that the 24 hour (or more) holter monitor is the best test for diagnosing irregular heartbeat, and that ECG's dont usually show anything because they are so short. Is it possible to just have the holter monitor test done before an ECG or any other test? Any information or advice will be much appreciated :) Thanks xx