Sudden blister on 5yo

[bamzooki]

Last night my 5yo DS appeared when I was going to bed, saying his finger hurt. When I looked, I found a blister, about the size of a 1p on his thumb, coming away from the edge of his nail.
I'm baffled about this - there have been no 'incidents' that I am aware of that might have caused it through heat or friction - and surely he would have mentioned it at bedtime? It's obviously causing discomfort given how many times he woke me last night, and he says it's itchy too.
Does anyone have any ideas what might have caused this? I'm sort of thinking it must be a contact allergy sort of thing, but again no idea to what.
And beyond covering it, should I be doing anything else in terms of treatment?

I've just checked again, and it has grown a bit overnight, and now there is one starting on the other thumb again at the corner of the nail. The skin looks red there too, perhaps not surprisingly.

Update for anyone else who might experience this.
Took him to docs as the blisters were just getting bigger and more angry looking, plus he started a temperature.
The blisters were caused by a bacterial infection getting under the skin, in his case round the nails where the skin had been pulled a bit. The big blister was drained because it was affected his knuckle joint, but the smaller one was left alone. So along with some antibiotics for the infection, he now has in dramatic looking bandage on his thumb to impress his friends with.

Now just have to tackle the red tape nightmare that is involved in getting school to administer antibiotics during the day....

that's interesting bamzooki, I saw this this morning but I didn't know what to say. It sounds very nasty. Glad you got him to the doctors.

Actually yes, it was quite alarming to realise how much it had got worse in between getting the appt at docs, and being in the surgery. It was very wierd.
I felt a silly going there to show the docs 'a blister', but I could tell by looking at ds's face that there was more going on. He just didn't look right, and I am very glad I trusted my instinct.

Was it a staph infection? There is one called mersa (mesa) a multi resistant strain of bacteria that is resistant to 'regular' antibiotics.
My ds was diagnosed with it yesterday. He doesn't have any sores though-perhaps acne? But we found his when he had a sore throat and had a strep test done=no strep but positive for Mersa staph infection. So perhaps he got his through surface contact and then touched his mouth or didn't wash his hands before eating (like during marching band-he is 15). Son was on amoxicillin for the week waiting for the culture to process, with no real improvement. He is now taking sulfamethoxazole.
I also have a 6 month old dd2 and she woke up today with a suspicious rash on her temple. She did get a couple of mosquito bites yesterday evening and those are regular round red spots, but 'rash' is like a thumb print.
So to doctor today and dd1 (14) says her throat is scratchy too-so a double appointment.
Off to disinfect all surfaces...

Oh and I hope your little one is doing better.

toomany - yes thanks he is getting much better - even if his fingers look like a lizard shedding it's skin.
It does seem to have responded to the treatment so I guess it can't have been a staph infection - he was given flucloxacillin (sp?). Th treatment is finished now, so I am just hoping the remaining redness is to do with deep skin healing rather then remaining infection, but I guess time will tell.
They didn't take any swabs etc to test for what strain the infection was anyway, so no good me asking!
I hope your little dd is ok, and that the older one responds better to treatment now. It's a horrible time of year for bugs I think.

I have only just seen this and was going to say it sound very simular to impegtigo, which we are battling here atm
glad his ok though