Ok, so she probably had a C-section then. Because, with most natural births, they cut the area from the vagina to the anus so there isn't as much tearing etc. Hence an episiotomy.
Anyways, if this just happened once, and it doesn't all the time, I wouldn't worry about it so much. The fact that she had watermelon probably made things move along quickly as well.
Embarrassing things happen during sex sometimes. I wouldn't make a big deal out of it. Probably makes her feel self-conscious.