6 month old light Sussex, very dirty bottom and red poo

[Elsachicken]

Hiya,
These chickens flew into our garden a week apart about 3 months ago, when I put pictures on here it was guessed at around the 3/4 month mark and a light Sussex. About three weeks her poo went red (not blood red) but I thought it was the multi vits in her water which has iron and red in colour. So I since stopped the vits to see if it helped, the poo looks slightly less red but the last two weeks Elsa's bottom has got very dirty!! She is eating and drinking lots throughout the day, energentic as usual. With the age estimates I received on here she is about 6 months old.
These are our first chickens we have had, and as said they were a surprise but they have been fab chickens and love having them
Here, we are total beginners though!
She has access to a soil pit to dig about and is constantly digging her straw and sawdust and making a mess!
Advice please , pics attached.

 

[coach723]

The vitamin supplement could have been the cause of the red. Many foods/treats can change the dropping color. For future reference, too much iron can be toxic (to anyone) and most supplements are pretty high in iron based on a chickens size/weight. Usually it is recommended to give vitamins that don't contain iron.
Various worms/parasites can cause runny poo's and dirty butts. If you can get a fecal float test done (many vets will do this even if they don't see birds) that would rule it in or out as an issue. I'd recommend you do that. Also dietary changes, or the kinds of and amounts of treats being given. Hot weather and drinking lots of water can also contribute. Sometimes dirty butts happen and there is not an underlying issue. If you find that you do not have a parasite issue, then adding probiotics to the water, or feeding small amounts of plain yogurt may help. Treats should be kept to no more than 10% of the diet.