Two of my colleagues work elsewhere, so our manager works out our shifts around theirs.
Sometimes she puts one of the colleagues shifts down before she has had a chance to get her other place's shifts, so I generally swap to accommodate her. She rings up at the end of every month, but has been ringing more often recently.
Two weeks ago she phoned me to swap some shifts which I did. I put them into my phone so I know where I am (starting to get forgetful these days). Today, she phoned me to swap more shifts that week (the week I've just done swaps in) 
She tried to pressure me and tried to dress it up as doing me a favour, but I just fobbed her off as I don't like this. I've swapped these shifts and don't want to go through it again.
One of these days I'm going to miss a shift at work because I'll be so confused about what I'm working.