Why does my heart rate get so high when I stand?

[JJstarr]

For about 3 months, my heart rate goes from anywhere between 70-90 bpm whilst sitting/lying down to 130-160bpm whilst standing. I'm aware that everyone's heart rate increases upon standing, but mine seems to stay up there until I have to lie-down. It's awful, I can feel it pulsing everywhere! I wake up often with more palpitations of around 120bpm too.

Alongside this, I have other symptoms that have been troubling me. I feel very lightheaded, my ears start ringing and wooshing, I am very nauseous and I sometimes collapse. I feel too hot in myself, like I'm burning, but my skin is cold.

Whilst I would have written this off as "just anxiety" since I have an anxiety disorder, I haven't really felt anxious since February and I can't remember the last time I had a panic attack. It's got to a point in which I can decipher between whether something is a panic attack or not, and this is not a panic attack.

I want to go and see a GP about this, but I'm concerned that they won't take me seriously due to my anxiety diagnoses. This has happened before with some stomach issues I was having, and I ended up getting diagnosed with eosinophilic colitis. Whatever this is, it's affecting my daily life. I can't get to my classes or meet up with friends etc. and it's bringing me down. Does anyone here know what this could potentially be, and what I should do?

FYI, I'm a 16 year old girl who eats and drinks a normal amount and is active.

[Red_Star]

Excerpt from Wikipedia's article Postural orthostatic tachycardia syndrome...

"The hallmark sign of POTS is a measured increase in heart rate by at least 30 beats per minute within 10 minutes of assuming an upright position.[2] For people aged between 12 and 19, the minimum increase for diagnosis is 40 beats per minute.[4] This symptom is known as orthostatic (upright) tachycardia (fast heart rate). It occurs without any coinciding drop in blood pressure, as that would indicate orthostatic hypotension.[2] It should be noted, however, that certain medications to treat POTS may cause orthostatic hypotension. It is accompanied by other features of orthostatic intolerance—symptoms which develop in an upright position and are relieved by reclining.[2] These orthostatic symptoms include palpitations, light-headedness, chest discomfort, shortness of breath,[2] nausea, weakness or "heaviness" in the lower legs, blurred vision and cognitive difficulties.[1] Symptoms may be exacerbated with prolonged sitting, prolonged standing, alcohol, heat, exercise, or eating a large meal."