Hi Elizabeth! Welcome to the Forum!
The flower you have pictured belongs to the "Campanula" genus. However, there are hundreds of species of "campanula", so I am not able to identify the specific variety for you. I have looked briefly for you through the ones I am familiar with, and I am leaning towards "Campanula rotundifolia" but yours may have a larger leaf. Check it out, and see what you think.
"Campanula" is a perennial, and so will continue to return every year. Also, some of them are self seeding or spread by roots, so be aware of possible spreading.
As to where it came from, these are the mysteries of gardening. A seed could have flown in with a breeze, a bird could have dropped a seed, a piece may have gotten in with another plant you brought home. There are many ways mystery plants appear, if you like it, keep it!
Feel free to drop in anytime with questions, a tidbit to add, or just to chat!