Hi.
Some times a post can get "lost" in a flurry of high activity on the forum. Didn't catch this the first time around but did when you gave it a bump.
I remember having braces and the irritation (practically cuts) they can cause to soft tissue. This one is a tough call. Some people do report having problems with mouth ulcers when they change over from smoking to vaping. See the list here:
http://www.e-cigarette-forum.com/forum/health-safety-e-smoking/3305-list-symptoms-when-quitting-tobacco-changing-ecigarette.html
In your case the braces complicates the question. The only thing I can think of if it's the braces, the irritation lines up with some point of contact with them. If you are having a problem with mouth ulcers that could show up in more random areas.
The topic of PG or VG allergic reactions comes up quite often on ECF. So much so that searching on the topic is kinda daunting. I coulda swore there is a list of possible allergic symptoms on ECF but I can't locate it. Some of the more common allergic symptoms can be nausea, headache, an intense feeling of itching on the skin with no rash or other cause present and difficulty breathing. I don't recall if mouth ulcers are ever reported in combination with any of those symptoms. I see an Allergist and he definitely thinks my experience - nausea, vomiting, itching and difficulty breathing - indicates I have an allergic reaction to PG. But allergic reactions in context of what we are vaping is an area with zero research right now. Allergic reaction is known to happen when PG is in FDA approved products - including anesthesia. Those get reported and tracked. But with e-cigs, for now, we as the end user are left in the wild to sort this issue out.
Hope this helps.