I have no idea really, I was going to suggest worms... Would it be worth taking a worm pill just in case? I'm wondering because I think that usually we do have a certain number of the little critters and so maybe the test for parasites is to test whether you have high levels of them. If you are unusually sensitive to them then it could be those even though their numbers are within normal range.

I used to take a worm pill every 6 months when I did a lot of work with my dogs. Because they did insist on licking my face and they seemed to take great pleasure in catching me by surprise and managing to get their tongue in my mouth. Ugh. I just picked them up over the counter at the local pharmacy. I don't know whether I actually ever had worms or not but once I thought I might have them I had an itchy bottom too (in the way I start scratching my head when people talk about headlice).

The itchiness... Do you scratch and do you end up sore from scratching? Do you have any other problems with constipation or diarreah (sp?) or anything like that? Do you eat spicy / hot food?

I'm not sure...

Do you have any allergies? Asthma? Excema? (sp?) Anything like that? If you have other allergies then your body could be extra sensitive to something... How about sugar substitutes? I'm not sure...