Hi.
A dimple anywhere on the breast does not always mean breast cancer,this can certainly occur from benign causes,but it should be investigated as the formation of a lump deep inside the breast tissue can sometimes pull the skin into the breast causing the dimple.
If you had a Mammogram 4 months ago, it could be that some changes in your breast have occurred during that time.Since this dimple is a new finding and you are worried about it, I would suggest to you to consult a breast specialist for evaluation,to determine what testing, if any, needs to be done.
Best wishes...
Forgot to add that I am very post menopausal.