Fruit Extract Causing Off-Flavor

So I'm fairly new to brewing (this was my fourth attempt) so I don't really have experience recognizing off-flavors. I recently made a darker beer and added some fruit extract (for 5 gallons I added about 3 tablespoons). The beer fermented fine, however there is a noticeable odd scent and flavor. The best way I can describe it is old rotten fruit. The beer isn't completely terrible, but it's by far the worst one so far. The extract itself smells fine by itself, but I was wondering if this off flavor definitely came from the extract, or if it could have come from another source.

What fruit flavor? What kind of beer is it? How was the extract stored?

A lot of people taste a chemically/fake/metal taste with extract flavorings. I can taste it, but it doesn't repulse me (especially after the beer is a couple months old). However, that doesn't sound like what you have. It's possible you added too much or the extract was old/bad or any other possible explanation.