It may be that the herpes (cold sore) virus starts reproducing and activates your immune system before you notice any physical signs. Your activated immune system releases cytokines, which activate your glial cells, which in turn increases the severity of your ME symptoms.
There's no cure for that herpes virus yet, although all the research for Covid vaccines might lead to one. There are some antiviral treatments that reduce the severity and duration of the herpes outbreak, but that might only shorten the time by a few days.
My cold sores generally trigger when the skin around my lips chill a bit. It sometimes starts up below a nostril during cold weather (the extra mucus cools the skin down even more). I think that virus starts up when it drops below normal body temperature. Maybe 'doing pretty good' somehow reduces your overall body temperature, or you end up doing something that cools wherever it is that your cold sores appear.
Oh, another possibility: for whatever reason, your immune system weakens a bit, which reduces your cytokine levels, which in turn reduces your ME symptoms. That weakness in turn allows the virus to start reproducing again.
I keep a food/activity/symptoms journal. If you keep one, you might be able to notice a pattern, such as a lack of vitamin C before the cold sores trigger, or maybe having some food that damages your immune system.
I find that applying moderate heat to the cold sores speeds up the healing. They usually clear up in a day or two if I apply heat.