Have you checked its temp?
Very low or very high it will want antibiotics.
Also consider it might need worming (especially if mum wasn't wormed prior to farrowing)
Is it eating, drinking and doing its business?
Loss of appetite in a pig is a sure sign that something is wrong. Try tempting with a saucer or syringe full of milk, it may just be sick of fighting its siblings to get fed and has given up!
Also i know the weather is quite warm at the moment but we had some piglets in the summer who were actually cold and similar to yours and once we put a heat lamp on them they made a rapid recovery.
HTH
Mandy