Over Pronation

I have been told by my doctor that I have slightly flat feet.
I play tennis twice a week and have done for the past 10 years.
Over the last 18 months I have probably gained another 1.5 stones.
So in all I am now 3 stone over my recommended weight.
I had no problem with my feet up until 18 months ago, but now when I do play tennis, or go for a long walk or am on my feet for hours at a time my feet ache and feel very stiff, especially for the first 10 minutes when I get up in the morning they take tiime to ‘settle in’ before I can walk properly.
My main question is would I have always had flat feet but am not feeling the problems until now because of my excess weight?
If I did lose 3 stone is it possible that I wouldn’t have these problems anymore, or will I always have flat feet now?
I really want to carry on being active, is this OK?