[Helen Wiltshire Horn]

Just checked my sheep and noticed that one of my ewes has a very snotty nose and is breathing heavily (which is probably the only reason that I noticed immediately as the snot is pretty clear-coloured).  No other signs of being unwell and she raced for her nuts with the rest of the flock. How do I treat this?  Jab of LA anti-biotic or is it likely to be viral?  I should add that she delivered a single ram two weeks' ago on Saturday (it was a straightforward delivery) and the ram lamb seems to be growing well.
Helen