My friend had this when her little one was around 10 weeks old. Was in hospital for something unrelated and they spotted a bruise on his foot which was apparently suspicious in a non mobile baby and my friend couldn’t explain how it happened. They had to stay in hospital for a few extra days with supervised visitation with their little boy. She was absolutely heart broken as she could only hold him when a nurse was there. Eventually concluded he’d kicked the end of the changing table during nappy change as he was such a wiggler (after he did the same thing when one of the nurses was changing him!). Whole thing took about 6 days to get straightened out and be allowed home, and then they had visits from social services every 2 weeks for 6 weeks to check for any other bruises. Hope everything gets sorted out for you, they’re just trying to keep baby safe even though it’s stressful right now.
I had similar when my youngest was around 1 (he’s 18 now so a long time ago). I was dealing with my eldest (2 at the time) when my youngest fell and bumped his head on the edge of the radiator. Took him straight to A&E and because the mark was an ‘usual shape’ (he’d hit in on the tap part of the radiator which didn’t have a plastic cover on it so the bolt was exposed, which I explained) they stripped him and checked for other marks (which there wasn’t any) and put in a referral to social services. They visited a few days later, checked basics - food in, they had beds, house was reasonable etc. Also showed them where he fell and the radiator which the exposed bolt matched the shape of the mark. They just gave me a ‘talking to’ about leaving him unattended (I was less than 5m away when it happened 🙄) then they closed the case as were satisfied with how his injury occurred. Try not to worry, if there’s no other concerns it’ll probably be closed pretty quick after a welfare check xxx
How old is your baby and what does the mark looks like? I know someone that had the same problem and then noticed the pattern when the baby would have fish she would get this red marks in her body, I would say request for an allergy test to be done with patches.