What Causes Soreness In Breasts And Irregular Periods?

I am female, almost 42 years old, and have tenderness (like pre-period soreness) in my left breast. I also think it is very slightly swollen, but I cannot feel any lumpsat all. The right breast is slightly tender, but the left moreso and it is localised around the left side. I have recently changed bra supplier and also have been very erratic periods, so wondered if either of these could be linked to it? Would be very grateful for some advice. Thanks, Rebecca