Extreme dizziness when standing up

35 female 6lbs overweight But otherwise quite healthy but take 100mg sertraline for anxiety Hi! Ever since being a teen I've been dizzy when standing up, but recently it's ramped up to feeling extremely faint and dizzy with a thumping heart when standing from lying or sitting. My resting heart rate is generally low around 55bpm but shoots up when i stand. I did a holter monitor and just got back the results, my GP says all looks ok to her but not her specialty so I'll get a cardiologist referral. Can anyone give insight into the results or suggestions of what the problem could be? CARDIOVASCULAR DIAGNOSTICS Holter Monitor Summary: Sinus rhythm/sinus arrhythmia. The average heart rate was 75. The minimum heart rate was 41 at 06:02. The maximum heart rate was 138 at 14:46. Rare ventricular ectopics. Rare supraventricular ectopics. No pauses greater than 2.0 seconds. Fifteen events; sinus bradycardia/sinus rhythm/sinus tachycardia, rates 54-128 bpm. Many thanks!