Hello,
In oily skin, the oil producing sebaceous glands are overactive and produce more oil than is needed. The oil oozes and gives the skin a greasy shine. It can be caused by heredity, diet, hormone levels, pregnancy, birth control pills and hot and humid weather.
Drinking coca cola does not cause oily skin.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.