This happened to me in my early thirties and my doctor said that it was probably hormonal.
I went on the birth control pill, ortho tri cyclen to be exact and this cleared it right up!
Even as an adult, fluctuating hormones can cause acne flare ups. Androgens are hormones released from the adrenal glands, the ovaries, and the testes. These hormones stimulate the sebaceous glands, increasing oil production and creating a skin that is more prone to comedones, or breakouts.
Adult onset acne most commonly affects women. Sharp hormonal fluctuations often occur during ovulation and menstruation, pregnancy, perimenopause, and menopause, and can also be caused by using certain birth control medications, others can help the problem. Women may see their acne suddenly develop, or worsen, during these periods of life.
It sounds like your diet is great. Do you avoid dairy as this can sometimes worsen problems? Also, sadly, chocolate and other sugars can aggravate problems!