When did your Neck Acne develop?

I have only been getting cysts on my neck over the past few months. What's weird is that 2 years ago, when the acne on my face was at it's worst (it was vile) I didn't have a single spot on my neck. Now I have them across my jawline and coming past my ear.

I think stress from exams may have caused them except they're still here 2 months after finishing...so when did everyone else develop neck acne and what do you think was the cause?