Green head fly bite area is getting larger over the last 2 hours and is red hot and itchy. When should I be concerned and request treatment?

Deer fly. That sounds like a deer fly bite -which can be painful. They can cause allergic reactions - and depending on your history of allergy you may need to be exceptionally careful. If it's not getting better, then you need to get evaluated. Warning signs would be feeling lightheaded, breathing problems, the development of hives, nausea/vomiting, rapid heart rate etc. Try some ice and maybe Benadryl (diphenhydramine) now.