Can anyone tell me what this rash on my chest is?

[WeAreEternal]

I have just noticed this rash across my chest.

I thought it could be heat rash, but it doesn't look like it to me.

Can anyone tell me what it is and how I can treat it.

Thanks.

Here is a better picture.

Does it stop at the edges of your clothes, or is it under them, too?

Some of it goes under but most is above the top of my tshirt.

Is it bumpy? It could be a reaction to sunlight called polymorphic light eruption. I get this. It is itchy. I can avoid it if I put sunscreen on. It only comes on bits of skin that have been uncovered, usually arms and legs or top of chest which is above the top of my shirts. It is different from heat rash.

www.patient.co.uk/health/polymorphic-light-eruption-leaflet

No it's not bumpy, which made me dismiss heat rash, the skin is not raised at all it's just blotchy.

I do have sun cream on today too.

It looks raised on your photo I must say - but I would second polymorphic light eruption too. Antihistamine cream may help.