Cucumbers naturally produce very bitter compounds known as cucurbitacins. These cytotoxins are bred out of the cucumbers by hybridization but they can always re-occur in plants that were cross-pollinated (usually unintentionally). Perhaps at one point you ate a cucumber containing some and you now have strongly aversive reactions to them?

(mostly just curious) Do you like pickles, any? Or does the cucumber flavor still come through?