Honestly I wouldn't buy a whole new load of nappies as it really sounds like she's grown a bit so that the nappies you've got aren't fitting quite right, or they've always been like that but you hadn't realised cos she wasn't moving around much. Although I assume she was crawling before she walked? Could it be that as she's now walking, her legs have slimmed down a bit so the nappies don't fit right?
I'd be more inclined to buy a couple of wraps for now and put them over your current nappies so you'd be using them like a fitted and wait a few weeks. Then try them again without the wraps and see if she's grown a bit more so they fit again. Basically, fitteds are just shaped nappies that aren't waterproof, so you put them on like normal and then add a wrap over the top. I would also try boosting your current nappies more. You said that she's a heavy wetter so you already boost them, but that doesn't mean that she couldn't outwee them still. As babies get older, they start holding their wee in for longer and releasing it all at once, so the nappy gets flooded quickly rather than little and often. If your nappies don't fit quite right and they're not quite absorbant enough, you would get leaks.