Pinworms are big in NZ, as we're an agricultural nation. Talk about ugh, not something I expected to have to deal with when I moved here.
Anyhow, my little nephew has had some serious growth issues and is a stick. They'd run all sorts of tests on him but it was eventually a naturopath who discovered that he had a major worm infestation. It's common to give prophalactic treatments every six months or so, but it wasn't enough. Apparently when there's an active infestation you need frequent doses to break the life cycle of the parasite, which makes sense. I personally think that some of his behavioural oddities can be attributed to this, as well. He was essentially malnourished from the extent of the infestation and that is never good for one's development, physical or mental.
If you're leery of MD's, then try the holistic healer route. Get thee to a naturopath or the like, they're pretty used to dealing with worms and from what I've heard, are more experienced than alleopaths in that area. But I wouldn't let this go on for too long, my poor little nephew was 5 and shorter than my then 3 year old (who granted is tall, but not THAT tall) and his ribs were poking out. They think the infestation had been going on for years. You don't want this slowly worsening the way he did.