She's pregnant.
You can see this because of what is said in Chapter 12. There, Darry says "When Sandy went to Florida ... it wasn't Soda..." We don't know what "it" is, but I assume that "it" refers to who the baby's father was. That's why Soda was so devastated.
We can also infer that she was pregnant because Soda offerred to marry her before she left.