Ditto Dat^^^I repeat...this is not a chick developing inside her.
Ditto Dat^^^I highly doubt this happened because of a feed switch.
Major ova release glitch in a 5 year old sexlink.
Like a really big 'blood spot'.
Notice the blood/veins are just on the outside of the yolk.
Could be disease, if it continues....or just a one off and you may never see it again.