Blood on egg

My 22 week old light Sussex has laid what may be her first egg and there is blood spots (a quite biggish - about the size of a 5p) in the nest box and she has blood under her vent. I don't think she has a prolapse and the egg is slightly large for a first one. Is this likely to be a one off or does anyone know if it is the beginning of a prolapse. I'm quite worried about her as she's one of my favourites!

That brings back memories - poor chicken having to do it most days! She seems fine today so time will tell. Thanks