Welcome to the forum Lisa. I am sorry your piggie is so poorly.
I would seek emergency veterinary attention, respiratory infections can develop quickly and it sounds like she is rather unwell. Allergies can cause similar symptoms but, to me her symptoms and with her being so young, I would get her checked out sooner rather than later. The sitting fluffed up is a common symptom of illness. Unfortunately the cost of having to just see the emergency vet will be high (usually around £40 plus just for the consultation!), but I would not wait on this one - in any case the cost will be the same as tomorrow.
Whatever you do, don't hand the pig back to Pets at Home, and I wouldn't trust their vets either, despite the "guarantee".
Another option is to find out if there is a rodentologist local to you - whereabouts are you? Rodentologists are not vets, but they are trained specifically and intensively in guinea pig health care.
Daisy should be given antibiotics - Baytril, most commonly - and ideally a one off dose of diuretic. With her being so young, the dose wants to be very small if this is given, certainly no more than 0.1ml, perhaps just half that dose. Piggies tend to get fluid in their lungs, as opposed to mucus, when they have respiratory infections and it can build up quite fast, so a one-off dose of diuretic should prevent the problem worsening. The Baytril will then have time to kick in and get rid of the infection quickly. Not many vets do give this diuretic straight away, so if the vet refuses to give it be sure to request it if her situation worsens.