My last menstrual cycle was only 10 days ago and i have developed extreme breast tenderness. what could cause this? is this symptom of pcos? not preg.

Hormones are strange: If it comes and goes in a few days, or at least before your next period, you can be pretty sure it's just that the hormones were a little off this month. But if you take medicine for psychiatric problems or it continues, it needs further testing

Tenderness: Please see your gynecologist as soon as you can get an appointment. If the delay is long consider a visit to the nearest Emergency facility.