Over Past Few Years, I Have Had A Frequently Puss/fluid

Over past few years, I have had a frequently puss/fluid filled painful blister , about 2cm in diameter, under the instep of my right foot. it usually lasts for several days before it bursts (usually in the shower). What comes out is thick and discoloured. Now, for the first time, I have developed a blister under both insteps. I don't have sweaty feet and I shower daily.