Pastafreak, did you support your kids whilst they were growing up, before they turn 18 (or there abouts)? I already know the answer to that question, I just asked it to make a point.
//But there's a kind of shame at the back of my mind that I 'should' be helping them...not the other way around.//
You have absolutely nothing to feel guilty or shameful about. Your daughter treating you will bring her joy - treating my own mum makes me happy, knowing that I can help her out if she needs it. It's not very often she does need help or would accept help, which is frustrating, that I can't help her out when she needs it.