I would either get your vet to come to you, or (making sure you use movement licences) take her to the vet asap - yellow fluid is either stomach bile or puss, either way you're better to get her checked out.
Did you worm the sow prior to her farrowing ?
Does the piglet have a temperature ? (it should be 102.5 F)
Does her nose seem okay ? No discharge, twisting of the snout etc ?
Is she eating and drinking ? (either from mum or on her own)
It could be any number of things - but the penicillin doesn't seem to be working (and I would have thought it should have by now) so time for the vet me thinks
HTH
Karen