Don't punish for potty training accidents, and don't assume "she knows better" because she's really doesn't and you're projecting your adult sensibilities on her. She is only 2 1/2 and is still learning . . . potty training can take many, many months to become ingrained.
Instead, you need to work on positive reinforcement. Try a visual system, which I found worked great with my older daughters (and I'm hoping will work well when my now 21 month old is ready to potty train!). We made a monthly chart, where sticker were placed on days they did everything right, and not good days remained blank. They were rewarded with a small item (usually something from a dollar store) after earning X (usually 6 or 7) amount of stickers in a row. They loved putting the stickers on the chart and seeing it fill up, and the empty days seemed to serve as a visual incentive to try harder. On those days, we would say things like "oh, now we have to start counting over again" but that would be it . . . no put downs, no punishement. And once they got through a full month (or more . . . you decide what works best for you), then a bigger, final reward was given (again, nothing too over the top, but something that really inspired them). We glued a picture of this ultimate gift to the chart as added visual incentive.
And I also agree with getting rid of the Pull-Ups, because they are just a glorified diaper. My oldest daughter (now 13) had pooping issues for almost six months, but not once would I put her in those things. I just kept using panties because I knew it felt worse have poop in her undies. I never used them with my now 9 year old and I have no intention to use them with my youngest. I know it's tough because it can be messy, but I think the "big girl undies" was also an incentive.
Good luck!