What is causing me to over pronate if my right foot arch is not flat?

Pronation normal? Most people pronate to a greater or lesser degree when walking if their hindfoot is supple. How much pronation is "over" is a judgement call especially if there are no associated symptoms. See an orthopedic or podiatric specialist for evaluation to see where you fit and the mechanics of your foot.
Pronation. This could take the available space. There could be a restriction of heel motion causing compensatory forefoot pronation . See your podiatrist for a biomecahnical explanantion and exam.
Genetics. The overpronation is not just caused by your foot, as this is also caused by the position of your legs that will create this condition or a possible previous injury to the right ankle, knee or hip. If this is painful, see your doctor. Good luck.