I was told I had thyroiditis.
Hi, I had a similar problem after my first child (she was around 1 when I finially went to the doctors) I was sent for an ultrasound then referred to a specialist. The problem did right itself without drugs luckily (hope yours will too) & I was closely monitored during my next pregnancy as they were convinced it would happen again (it didn't!) I was told I have about an 80% chance of a thyroid problem later in life as I had this problem & my mum has an underactive thyroid.
It hasn't played up during my treatment for BC but I was lucky enough not to have chemo or radiotherapy. I've been on tamoxifen since October 2017 and apart from periods going irregular (only from last month tho) I've been ok. I'm 47 so it could be the start of menopause or the drugs!!
Hope your thyroid sorts itself out - it's a horrible feeling! I eat dried cranberries as I have heard they are high in iodine & that's good for thyroids so I thought I'd try it!! Google foods high in iodine & see if they help.