ankle discoloration

My mom is 56 yo and overall healthy. She has noticed that both ankles have developed a tan uneven pigmentation (similar to age spots). There is no gross swelling or pitting edema appreciated - However, she does note occasional soreness in her ankles (not on skin).  (She is generally healthy, does take lower dose nifedical, hctz, and metoprolol - for blood pressure x several yrs).  After discussing with a Dr. - he thought that perhaps the nifedical was causing these sx...and placed her on furosemide (and potassium), as well.  
#1.  Do her sx seem consistent- as a side effect of med?
#2.  If it is a sign of edema...can this occur without appreciating the swelling component?
#3.  Why add furosemide...isn't HCTZ also a diuretic...(would increasing the dose have same effect?)
#4.  Can there be another reason for this discoloration?