Red, hot and limpy without obvious trauma - same day appointment. Please call your GP surgery.
I called NHS 111 and they pretty much said the same. I ended up in the local walk-in centre (hobble-in centre, more like!). The nurse there said it's almost certainly an insect bite, not serious, but to keep an eye out for various symptoms, and to try not to touch it as that increases the histamine reaction. That of course led to a doh! moment as I have histamine intolerance so of course my body is going nuts.
I felt totally wiped out yesterday, feel a bit better today but area is still very red and swollen. Paracetamol and anti-histamines are my friends. Sigh.
Sorry to hear of your tuna tin accident, Socks! Tins are the devil.