It sounds like one of the thread tensions is too tight or you are zooming it around under the foot when sewing the circles together and pulling at all the bias. Starch your circles, reduce the pressure on your foot if possible and go slower if that is the problem. I think the bias problem is more likely but when you turn them right sides out again, you might be ok. Starch them and try not to tug on them very much while sewing the circles together.