Hi. Adult acne is becoming more common and can occur on the face, chest or back. Androgens (male hormones found in both male and female bodies) stimulate the sebaceous glands which produce oil. If you have high levels of androgen, or a particular sensitivity to their effects you may produce too much oil. Your glands then get blocked with excess oil and bacteria and spots form.
The most likely cause is hormonal change. Is there any chance you could be pregnant? Or have you recently changed your method of birth control? Women prone to acne will benefit from taking a combined contraceptive pill as oestrogen blocks the effect of androgens on the skin. Avoid the mini-pill, depot shot or hormonal IUD as these can induce acne.
Diet has not been shown to effect the abnormal skin processes involved in acne. Some people are just more sensitive to the effects of androgens than others.
If this is diagnosed as acne your doctor can prescribe an effective treatment. Topical synthetic retinoids like Differin (adapalene 0.1%) are effective and generally well tolerated. A course of oral antibiotics such as Oxytetracycline or Lymecycline may also be indicated.
Try changing your shampoo/creamrinse, soap, lotion. I find some of these products (especially highly perfumed ones) flare-up my bacne.