I recently had a similar thing happen with me. In my case, taking Lithium cased my thyroid to go a bit crazy and I have a minor version of hypothyroidism. My doctor wouldn't even have noticed anything was different until I came in saying I kept bruising very easily.

I wouldn't knock anything or run into anything, I wouldn't be jabbed with anything, etc but I would get horrible bruises. It actually became a regular thing to check my entire body after showers to see if there were new ones.

My CBC showed only a slight decrease but it was enough that I was diagnosed with hypothyroidism. Could it be a possibility? I'd say get another blood test and tell them to look specifically for any kind of thyroid abnormality, big or small.

My options were to basically come off Lithium, see if it corrects itself, or to stay on Lithium and take Levothyroxine.

I'm not a doctor but that's my own experience, sounds very similar to yours and might be worth asking about.