Sour Grapes!

It was supposed to be a great mooment -- I had waited weeks for my baby watermelon to grow. The entire outside was dark green so I finally picked it. We cut it open ... the inside wasn't bright and pink but I didn't want to be judgmental. I took a big bite -- it was ICKY. GROSS. HOORIFFIC. My baby watermelon was sour! :( I won't lie -- I was very saddened. I wonder what I did wrong? Any idea?
Luckily there is another one growing. Knock wood that this one will be sweetness.