Thanks for your replies.
We took her over to the vet today, and he didn't mention it, so I think it must have been a gap in the feather clump.
Very odd.
But she had definitely taken a turn for the worse yesterday. Today we got her an appointment for 4:30 and by the time that came round it looks like she'd hardly eaten all day, when she sat on the vet's table she wouldn't support her own weight (making a claw).
He said he could feel a lump which could be a tumour or could be a stuck egg.
So we've had her put down.
he said he could give antibiotics but it probably wouldn't work, and that she seemed in pain.
And apparently they're not allowed to give baytril anymore!
It wasn't fun explaining the process to littledon - we told her they'd put her to sleep and then she'll die when she's asleep.
The lady at the desk gave her a little butterfly on a card, that had seeds in it, so she sowed that this evening.