What happen to my heart yesterday?

Yesterday out of no where My heart went crazy. It suddenly started beating very fast 248 and I had dizziness and shortness of breath. I was rushed to the ER and given Meds to bring down my heart rate. My labs all came back normal no cardiac enzymes and my ekg looked good. What could have triggered this? I'm 43 and I am overweight. The only lab that came back out of whack and it was WAY out of whack was my Thyroid .. my TSH was 276.. Normal would be .4-4 so even though I do take 200mcg of synthroid I was severely Hypothyroid . Any ideas?