Am I allergic to watermelon juice?

While travelling recently, I drank watermelon juice that I believe was bad because it caused my face to swell, itch, and made it difficult to breathe for the first time in my life. HK
Dear HK
It seems like you are experiencing symptoms of oral allergy syndrome, which typically affects older children, teenagers, and young adults who have been consuming certain fruits or vegetables for an extended period without any issues. It usually takes a few years for these allergies to develop.
Symptoms often occur immediately after consuming raw fruits, vegetables, and nuts that contain proteins causing reactions, which are usually limited to the lips, mouth, and throat. 
The stomach acid typically breaks down the proteins, preventing further spread of the reactions. The specific foods that trigger reactions vary from person to person, but in your case, it was watermelon.
While these reactions are generally not very serious, continued consumption of watermelon is likely to lead to a more severe reaction known as anaphylaxis, which causes breathlessness and can be life-threatening.
Based on your medical history and test results, doctors may recommend avoiding certain foods, especially watermelon. 
For other foods, cooking or baking them may help reduce the risk of a reaction. If a reaction occurs, antihistamines may be helpful.