I stopped
fainting/feeling faint after that. Before
the fainting spells, though, I also took an annual
flu shot (I've taken it the year before in 2018, as well, and had no issues). My blood glucose levels were within normal limits (although they were all taken when I felt no palpitations and several hours after eating).
Tests for my thyroid (including TSH, thyroglobulin, free T3, free T4, T3, T4) all came back normal.