I totally hear you. The same thing happened to us.
First of all breathe. This is a phase and will work itself out. But you might have to intervene.
My son potty trained at 2.5. The process was fairly easy, but we did have some accidents. Then when he was about 3.5 (2 months ago) he started having major accidents. Like he would poop completely in his underpants, and not have any left to get in the bowl. It started happening 3-4 times a week, and he had pee pee accidents too. He seemed very unconcerned about it all. In fact he wuold tell us "just go and get me a new pair of underpants". We knew that he would play too long and not make it to the potty in time, but nothing seemed to motivate him.
And just like you I was physically stressed to put him back in diapers, so afraid that it would undo all the efforts we had worked so hard on. I was emotionally stressed about this. I worried that I would not put him into school in the fall. I worried about everything. Everyone said don't yell, don't talk about it, don't shame him. Keep it light and no big deal. But that just didn't work for me. In the end I did put him back in diapers. And specifically in his 18 month old brother's diapers (not the big boy pull ups that he wears overnight). He completely understood the difference between his pull ups and the 'little baby diapers'. But I told him that I would not let his brother run around in underpants because he was too little and not ready for underpants. And I said, if you aren't ready to get to the potty on time I then maybe you aren't ready for underpants either. He was sad and felt bad about himself. I didn't yell, or truly shame him in a mean way. It was one of the hardest decisions I have made as a parent.
But low and behold. It did the trick. He started getting to the potty on time and he now had a vested interest in not having accidents. It happened maybe twice in 2 months that I put a diaper on him and both times I had the same result, he got better.
In regards to your situation, it sounds like she is a little anxious about the potty because she is going so often. But don't get too stressed about the pull ups. It might be just the motivation she needs. Talk to her about the pull ups, give her a chance to pull it together before you take action. At least then she knows and you won't just be springing it on her. My son always wait until you take action--threats don't work, only action. But I have always believed that girls respond more to words.
Good luck!! I promise you--this too shall pass.