Onset of lay can cause more runny butts at first as the vents are more "open" and the poop doesn't have that nice, quick delivery that it once had with the nice, tight vent. The only bird I have laying right now has a black streak down the middle of her white bum for the same reason. Granny always said the birds with the messiest butts were her best layers and I believe her...those with the largest eggs and most frequent laying seem to have messy butts more often than the birds that lay less steady and lay smaller eggs.
Go out tonight and take that bird off the roost and give that butt a whiff...if it's a strong, almost like a dead animal odor, it may be gleet. If not, and it just smells like plain ol' chicken poop, I wouldn't give it a second's thought. If it's gleet, I'd just cull her...if she develops something like that on the good probiotics you are feeding, there is no helping her at that point unless you medicate and be prepared to medicate her again when it comes around again.
Look at her vent..is it nice and loose, open a bit? Could be the cause and due to laying steady.
Back off on the amounts of FF you are feeding and see what happens..they may just be getting more than enough nutrients and need less total feed right now. Feed amounts fluctuate through the seasons, so be ready to adjust those according to flock needs.
Please let us know how this goes along? It should be interesting to see what happens...
Hi Bee... Short update on this; I went out to the coop after dark last night and got ahold
of the BS and one of my BR's ... Gave both their butts a whiff and it didn't seem abnormal to me as far odour. As I stated, they have some messy butts and the BR's is a yellowish/whitish streak down her backside. Both birds that I grabbed seemed lively, energetic and I even got another peck from the BS. I guess I'll continue to wait it out and see what happens. I'm hoping it's just the egg laying that has caused their vents to be loose or maybe an adjustment to the fact that a new rooster has been introduced to the flock. I did notice the rooster pays a fair amount of "attention" to the BS... Could this be reason for her to distance herself a bit from the flock so that he doesn't bother her as much ?