Hi Kerry
What a lovely gift you got today!
While I'm not sure I can give you a definite answer to your question, my instinct says yes. I don't think things like that are coincidental. I think it was Shandy's way of telling you that she's okay...she's over the Rainbow Bridge! She's running free...free of all pain and age. She's watching over you and sent you a message to comfort you.
I had waited weeks for a sign from Jasmine. I was getting pretty upset when none came. Then, I got two on two separate occasions. The first one was subtle...a thing of nature like your rainbow. There was a small cluster of dandelions growing where Jasmine like to stand in the back yard, near these evergreen trees/bushes. They were all alone, which isn't common for dandelions. Being weeds, they tend to spread fast and take over an area. But these few were the only ones. I felt it was Jazzy's way of sending me something beautiful to tell me she was at peace.
Another time, I was sobbing in the back yard...and I whispered to her "please send me a sign because I miss you so much". A moment later, a mourning dove landed on the grape vine holder. I knew it was her. I talked to her and then when I said "fly free baby", it took off. Mourning doves have always visited me when someone close died, including my childhood dog years ago. So I KNEW.
I haven't received any more, but if I never do, I'll always be grateful. Shandy is with you and I'm sure she'll send you something else when the pain isn't so raw.
Hold onto that memory of the beautiful rainbow...it reflects her beautiful life.