Do you know, they say imitation is the greatest form of flattery! I wouldn't let it worry you, truly. The thing is there are very many people out there who simply can't visualize art for themselves, but know what they do like when they see it, and are able to copy/replicate it and produce something they enjoy. They absolutely ought to give credit to the original artist in their work, but most often they don't, and one can just hope that they are honest enough not to claim it as their own original artwork. When one publishes anything in whatever form of media, it suddenly seems to become public property, and no amount of saying "artists' permission" "copyright" seems to make a difference. So, it's simply easier to accept that it happens and be glad that others enjoy your designs enough to copy them and make something for themselves or others that they will appreciate, and consider an acknowledgement of your original artistry a bonus.
I don't think you should worry too much. I think there are enormous amounts of stuff that have been copied by someone. I see things that I say to myself "i was making those 40 years ago' . I made pillow cases a bit like yours for my granddaughter about 20 years ago. I have been making all sorts of bags for donkey's years. There were no patchwork shops and I used to buy fabrics at the local draper shop. Back in the 1940's my mother had a dressmaking business mostly mmaking babies clothes. I used to watch her drawing up her designs. They featured lots of smocking. I'm sure those patterns have been copied many times over the years. I like what you make . I think copying becomes more of a problem when overseas companies copy something to make big profits. Enjoy what you do and be happy that others like what you do. All the best Judy.
I would be enormously flattered that they liked your design so much...;)