Need work advice regarding promotion

Baby is due 2/21/19. I was recently handed a HUGE promotion (no application it was just dropped in my lap) to manage an entire new department along with managing 2 additional supervisors. If I wasn’t leaving for 12 weeks soon I wouldn’t hesitate to ask for a raise for handling all these new responsibilities. But I feel because I’m about to take extended leave, I am not in a position to ask for more compensation. Everyone I’ve talked to believes I should get things set up for the department to run smoothly while I’m away and then ask for additional compensation when I return. But I basically feel like I’m being taken advantage of for the next 2.5 months til I leave. Opinions?