Nightly rash - any idea what it could be?

[Muchtoomuchtodo]

For the last 3 nights at about 7pm DS has complained of feeling itchy. When I've looked he's had about a dozen raised red spots mainly on his tummy, but a few on his back and at the top of his arms.

I've given him piriton and he's gone to bed, slept well and the spots have gone by the morning, only to reappear that evening.

They come before he's had a bath or shower. I'm trying to add a photo but it's on my phone so I think I'll need to do it separately

Any ideas what it could be?

Here it is

Heat rash? Do they come up if he has no bath?

Oh sorry just seen post properly. No idea sorry.

Could he have developed an allergy to something he's eating?

I can't think of anything that he's eaten that's the same for the past 3 nights (I'm assuming that lunchtime allergies would have shown sooner?)

Wednesday was curry, Thursday he had meatballs at Ikea and tonight he had jacket potato with ham, cheese and beans.

He's fast asleep now. It only bothers him for a short while after they come up and before the piriton works but I don't want to just keep giving piriton. The advice leaflet with the medicine says if the problem persists see your Dr, it doesn't give a maximum amount of days that I can /should give it for.