I have hypothyroidism and 6 months after having my eldest i began feeling awful, just as you described. Despite having previous normal thyroid levels after birth i was feeling exhausted, weak and having regular palpitations. Went to gp who dismissed the palpitations as being associated with thyroid problems and insisted on doing ecgs and all sorts. When these came back normal he then told me it was stress !? Got a second opinion and blood tests showed a significant dip in thyroid levels. Once my meds were increased i began to feel human again and palpations stopped. Although palpitations is not typically a hypothyroidism symptom, it is of postpartum thyroiditis. Look it up for more info and go armed to your gp. Good luck, i know exactly how your feeling and it is rotten but you will feel 100% better once meds have been adapted! x