Take my thoughts with a grain of salt because I don't have 3 kids or daughters.

Follow your instincts. If you think there is something there, then get it investigated. If there is no issues, then at least it is ruled out.

Also, as she is the middle child, is she trying to get some attention? Was she like this before DS was born? Does she feel like she is left out a little?

Does she favour time with you or DH more? Can a reward be some solid time with the parent she wants to spend time with the most?

Not much help as I am sure you have already thought of this.