I spilled lemonade on my carpet, after cleaning the stain reappears as a brown stain a few days later.?

It is like the stain has oxidized and turns brown. I called the manufacturer of the lemonade and it is just citric acid, sugar and lemon juice. I was having some construction down a few days after the big spill (I fell down the stairs) and thought I would leave the stain to clean it after the construction, so it was left about a month. The construction mess cleaned up but the lemonade keeps coming back, thanks for your help!