I hope you have seen a doctor (Endocrinologist)about this.
I suffer from an autoimmune thyroid disease called "Hashimotos Thyroiditis" which causes both hypothyroid, (under-active) and hyperthyroid, (over-active)states. My diagnosis was confirmed two years after my daughter's birth. I suffered from skipping and racing in my heart, lump in my throat, nervous jitters, intolerance to heat, intestinal spasms, severe agitation, sweats, followed by severe mental and physical fatigue, chills, brain fog, memory loss, dry skin and hair, weakness, and dizzy spells. I have to take Synthroid (Levothyroxine),every morning two hours before I eat which makes a great difference. Hopefully you don't have this same condition, but doctors have told me it is quite common in women after childbirth. It is also very important to see a doctor about this before you plan on becoming pregnant again so it does not harm your unborn child.