You would probably have blisters by now, and a red patch isn't likely to be herpes.
What you have sounds like a fungus.
Does your Canestan say Canestan HC or just Canestan? If it's HC, that means it's hydrocortisone, which is a steroid, and herpes doesn't respond well at all to steroids. It would likely make it much worse.
Is this from when you received protected oral? The chances of you getting anything from that are extremely low.
Canestan is good for yeast, but might not be good for other fugal infections. If it doesn't improve in a day or two, see your doctor.