Since your hen is eating and drinking normally, I agree with you that it might be due to pecking. Where is she located in the pecking order?Yesterday I noticed my PR hen face half of it was swelling. I left it alone bc she was eating and drinking and doing her normal routine and thought maybe it was from pecking but this morning her swelling got worse. It’s only on the one side and I don’t know what to do. The rest of my flock is fine.
If she is at the bottom of the pecking order, you have your answer. You may not see other birds pecking her, however, when they go to roost for the night, that's when the pecking can occur. Chickens have a pecking order when it comes to roosting at night.