I've been back from maternity leave a couple of months. Been full time before but came back to 4 days and slightly smaller workload. One of my colleagues is about to start her maternity leave and I've been asked to cover it. That would mean that I'll be covering a full time position as well as continuing with my normal duties. Nature of the job will mean that it will be relatively quiet over the winter months but I still feel like it's a bit much to ask? No mention of pay rise of course. Everyone just assumed that I'll be doing it as my workload is not quite enough for 4 days. I can't decline to do it but should I ask for a pay rise?