A single or few episodes of grey green stools isnt anything to worry about.
It is usually caused by some food that you took. The most common reason is a lot of green vegetables. Food coloring in some foods can also cause this. The third cause is iron supplements. These are the most common causes.
Green stools can also be caused when stools pass rapidly through the intestines, for some reason like perhaps a bad food item.
In your case, iron supplements are ruled out and also rapid passage of stools through the intestine.
So you should carefully consider your diet and whether you have been taking an excess of green vegetables or some supplement that might contain green coloring.
The other step that you should take is to take Probiotics. This may also be due to disturbed intestinal flora due to the surgery. Probiotic capsules which contain healthy intestinal microrganisms will restore the normal flora and will help you.
You should discuss about Probiotic capsules with your pharmacist and choose a good brand, there are several brands available.
Dont worry about this, it does not signify any major problem. Check your die and also take probiotics, this will usually be sufficient to make your stools normal.
You will need to approach your doctor if there are several such episodes even after you have ruled out any items in your diet.
You can look up here: http://ibdcrohns.about.com/cs/otherdiseases/a/greenstool.htm
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