Surgery for axiliary breast tissue in armpit 7 years ago, it seems to be growing back. Is it just skin or tissue growing back?

Weight change (twenty pounds is substantial) and just getting older can affect the size of your breasts including axillary breast tissue. Try weight reduction and see if this is a simple non surgical approach to your problem. 

It is impossible to say without an in person examination. Please speak with your plastic surgeon to address this.

 Based on your history, especially that of weight gain, it is likely that you are dealing with some breast and/or adipose tissue in the area. This tissue, may be hormonally sensitive and  therefore, tender. Your plastic surgeon may be able to remove the “excess” skin/tissue at the time of your breast surgery, if it is of significant concern to you. Otherwise, besides appearance wise, it should not affect your planned surgery in any way. Best wishes for outcome that you will be very pleased with.