Hi XXXXXXX,
It seems that this could be a functional disorder of the bowels. This is usually a diagnosis of exclusion, after other tests are unrevealing. The fact that this happens only once a month makes it highly unlikely that something is going to pop up on any test.
By functional disorder I mean that the bowels may occasional contract more rapidly causing you to have diarrhea. This is particularly common in diabetics. After suffering from diabetes for many years the nerves in the body can get damaged, and the most common manifestation is peripheral diabetic neuropathy (when you loose sensation of your feet and hands), however you can get bowel neuropathy as well. Keep in mind that the most common gut neuropathy is called "gastroparesis" (delayed gastric emptying), but you could also get diarrhea as you describe.
There is no specific treatment, and recommendations are to take antidiarrheals as needed (such as loperamide or lomotil).
Hope that helps, let me know if you have any further questions!

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I would suggest you to take loperamide as needed. You can also take it before any situation that you would like to avoid to move your bowels. The effect of this medication is fast (within 30 minutes), and it wears off the body after a few hours.
If loperamide doesn't work, then other medications can by tried such as lomotil, lotronex.
Take a high fiber diet, or add fiber supplements to your diet (such as Metamucil). Limit the amount of dairy products (milk, yogurts, cheeses).
Also given the fact that you are diabetic it is also possible that you use artificial sweeteners (such as Splenda, Sweet n low, etc), so keep in mind that these are good for your diabetes but can cause bloating and diarrhea, therefore you should decrease the amount of those if you indeed use them.

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