Has anyone ever heard of a vaginal sore being cause by thong irritation or from sanitary pad use?

I once had a small sore because i had been wearing a pad for about two and a half weeks straight because my period was late a didn't want to have an accident. I had also been doing a lot of walking during this time and therefore sweating down there at the same time. It was painful and red but not very large. My doctor agreed that this was the cause and it went shortly after I stopped wearing a pad. I would guess that a thong could have the same affect.

Yes, thongs can cause friction damage and menstrual pads can cause irritation.

If you use commercial menstrual pads - for example Always or Kotex - then this is a fairly common issue because these type of pads have rough edges that can rub the skin raw. These types of pads prevent air-flow so make genitals sweaty and stop blood from drying so this cause cause the skin to stay moist and become irritated, a little like diaper rash if left too long it can result in sores developing. Commercial tampons also contain chemicals you may be allergic to.

It's best to see your doctor to check for certain that this is all that is causing the sores, but in future avoid thongs and rather than commercial pads opt for organic pads or better still cloth pads, you can also use options like menstrual cups or softcups which are internal and don't require pad use.