Another mistake I made last summer was also in a hotpad. I don't prewash anything. The only time I wash a fabric is when it gets dirty somehow.
In 11 years I have never had a fabric run. I spray most of my stuff with spray starch when it is done.
I had made this hotpad and was pretty satisfied with it til I went to spray and iron it. It turned several of the white sections to pink!! I tried to wash it out. I was not going to redo it. So I decided to see if I could turn all the white pieces pink. I did get it to do that but I have never been happy with it. It looks deliberately done and not done well to me.
The red that did this is one of my favorites and I still use it all the time. I found that that one particular piece of red was the only one that ran. None of the rest of them ever had a problem.I guess it was just that one lot that had a problem.
I still do not wash my fabrics. Too much time and trouble and I prefer the state they come in when I buy them. I don't want to go through the hassle of washing and spraying and ironing to get back to the stiffness they have when new.
This is the one and only time I have ever had this problem.