Trouble pumping but fine feeding?!

I'm trying to express the odd bottle of breast milk just for storage purposes, in case we need it. I've noticed over the last few days that I can feed absolutely fine but can hardly pump any milk off. I've used hot flannels, physically held baby whilst pumping one breast, etc. Is this fairly common? It just seems that my body seems to hold onto milk for the baby!  I'm in the UK and have a year off so don't 'need' the milk. I just thought it would be good to have some kept aside as an emergency measure or in case we're out and it's convenient to take a bottle of breast milk with us. Any suggestions? Do I just leave it and enjoy feeding baby?