Watermelon problem-what went wrong?

[futurowoman]

Hi there! I live in the Dallas-Fort Worth, Texas area, and I'm growing watermelons for the first time. The variety I'm growing in Black Diamond. I started them from seedlings I got at Lowe's. Today I harvested my first melon. It appeared ripe according to these signs:
1) yellow bottom
2) stem splitting/getting dry
3) sounded hollow when thumped
4) appearing more dull on the rind.

Here's the melon right after I picked it.

[img]https://madagascarmonegasque.smugmug.com/photos/946222719_UookN-M.jpg[/img]

However, when I cut it open, it smelled very ripe, and the consistency of the red flesh was like a very ripe watermelon, but it was bitter and had channels of rind running through it:

[img]https://madagascarmonegasque.smugmug.com/photos/946222750_mEpHn-M.jpg[/img]

It doesn't show too well in this photo above, but it's almost like there were channels of melon, with rind in a star-like shape inside. There were three sets of two "petals" in a pattern in the melon.

Does anyone know what I did wrong? Did I merely pick it too soon, or is this something else? The melon seemed ripe, and the stem was splitting from the vine/drying out. I have other melons on the vine, so I'd like to know what I did wrong so that I can get ripe ones next time. Thank you so much!