I've never had runny noses, but a few occasionally go off their food and look a bit sorry for themselves for a day or two. They go off their food - come out of the ark looking interested, but sniff, have a pee, and then go back. Personally, I would give a series of shots of penicillin (eg Pen and Strep) if you're worried.
It could be just a passing thing. In which case, just make sure the piglet is feeding okay.