Hailey Bieber might have found herself singing to husband Justin’s hit 2015 song “What Do You Mean?” after discovering that back pain is one of the biggest complaints from a pregnant woman. On Wednesday, June 12, the gorgeous model took to Instagram to admit she’s been suffering from some discomfort while carrying her and the “Baby” singer’s first child. “So who was gonna tell me about the lower back pain?” Hailey wrote alongside a photo of herself lounging on a chair outside with headphones on. In the snap, the brunette bombshell had her hair slicked back while her adorable baby bump peeked t…