Strange red mark on toddler

[Crazylou]

This mark been there for few hours and won't go away not even pressed anyone seen this before

Looks like he's been lying on something, or his clothes are too tight.

That's what I thought he's only got a tshirt on and has been sitting on the couch

Is it dye transferred out of a fabric or something?

He's has nothing else on and has been sitting on the couch for a few hours?! Is he poorly?

Was he laid on the couch?

Is the marking raised or hot?

Antihistamine?

If that's a reaction rash it could be urticaria, and a toddler will need to see someone just in case of swelling.

The treatment is usually anti-histamine like piriton.

Black tshirt, been up and down so not really been sitting for hours, he's whining on and off probably due to tiredness as he's not napped, he's always warm it doesn't feel any hotter than rest of body, it's just strange it's just appeared

He's got IGg antibodies problem and is attending pediatrician this week for more bloods to be taken,

It's just where the sofa fabric has pushed into his skin slightly.

I think it is unlikely to be from the fabric from the couch or t-shirt. It looks a bit like urticaria but that tends to be itchy and it doesn't look like he has been scratching.
Based on his UGh issues I would probably prefer him to be seen by a Go. I have no idea what it could be but in case it is a symptom of something noteworthy it might be better safe than sorry.