Constipation and corn syrup

My LO is 6 weeks old, she usually has at least one big poop per day but hasn't gone for more than 3 days and we believe she is constipated, nurse practitioner recommended corn syrup in her bottle, wondering how long after I feed her that I should expect her to go? Gonna schedule an apt with her PCP tomorrow if it doesn't happen today :( been reading mixed responses on using corn syrup, any other tips for babies this young??