Postpartum thyroiditis is pretty common among new moms, often going undetected and affecting at least 5-10% of postpartum women during their first year PP. Sometimes, the mom will go hyper-thyroid for a while (which is currently happening to me) and will lose weight quickly but have a lot of other uncomfy symptoms too. This phase typically leads to a hypo-phase where weight loss is difficult and many women skip the hyper-phase and go straight to being hypo-thyroid, which may be you.

You should have a simple blood test that will confirm if your thyroid is not producing enough hormone, which is possible considering your difficulty losing weight, especially considering you were breastfeeding and restricting your intake. If you turn out to be hypo, the condition is most likely temporary and, in about 2/3 of the cases, will return to normal within a few weeks or months. Also, your doc can give you hormone replacement if you need it so that your metabolism will speed up to normal.