Need help figuring out the cause of transient T wave inversion

[pl535n]

Hello everyone,
I’m a 22 year old female in remission from cancer.
In February 2022 I experienced some heart issues cause of which was never found out. Multiple cardiologists told me they have no idea what it was. It started with inappropriate sinus tachycardia and occasional chest pain. Those issues became pretty unbearable, making it hard to function due to the tachycardia. I went to the ER, they took an ECG that revealed sinus tachycardia and flattened T waves in lead II, after taking a chest x ray i was discharged since  there wasn’t any other apparent issues. I waited a couple days but because my condition didn’t improve I returned to the ER. Another ECG was taken, this time it revealed inverted T waved in lead II and biphasic (and some flattened) T waves in leads V2-V6. I was admitted to the hospital for 4 days during which an echocardiogram showed normal results, and I underwent a bike stress test. The stress test couldn’t be properly completed because my sinus tachycardia was making it hard to cycle the required minimum 5 of minutes. From what they did record during that test they couldn’t find an issue either (T waves stayed inverted in lead II and biphasic/flat in V2-6). I was discharged home with a follow up in a week and a scheduled heart MRI. The follow up ECG showed that both the sinus tachycardia and the T wave inversion were resolved. Later, the MRI showed completely normal results without signs of pericarditis or myocarditis.
Does any one have any clues on what the issue could have  been? It makes me anxious thinking it could happen again