There was nothing in the blood tests to suggest a cause. My iron (blood serum ferritin) level was 44 ng/mL on a scale where 14-150 ng/mL is considered normal for females. I am not an expert, but apparently this means I'm not anaemic. I do remember friends with anaemia having ferritin levels in the 17-19 ng/mL range.
Also my TSH level was 1.93 uIU/mL - which is increased from May but still within the "normal" range. I am definitely not hyperthyroid because of too much thyroxine, since that would give me a TSH level of 0.3 or less!
I forgot whether they checked my vitamin D level, but I've stopped taking the supplements for now just in case they were causing palpitations... it's been more than a week without them, and my heart seems to be getting worse. Eeep.
Being neither anaemic nor hyperthyroid, there is no convenient explanation for the tachycardia. I am resolutely not Googling causes of tachycardia because I'm worried enough already. There's something about chest pain and knowing it's your heart which is just terrifying.
I just don't understand how a part of my body which has never caused me any trouble before could suddenly start scaring me like this. Especially since it's not as if the rest of me is healthy...