Further to Stumpy's post, that pink circle most likely came from partially transparent pixels on the edge of your circle which blended with the magic pink color when you merged layers, or "flattened" the image to save it as a bitmap file.
My guess is you created your circle either as a vector graphic which uses antialiasing to create a smooth line, or you had antialiasing enabled on your raster drawing tool. That antialiasing is accomplished by blending pixels at the edge with the background image.
If the background is magic pink, those pixels at the edge blend with the magic pink color, leaving them no longer a true magic pink color. If the background is transparent, those pixels become semi-transparent. Then when you merge that layer with a magic pink background, those edge pixels blend with the pink layer beneathe with the same result.
This unfortunately is a drawback of using bitmaps which cannot be avoided. The best way I have found to do it is to make sure I have only 100% opaque pixels in my graphic before merging with a magic pink background layer. Hippy had posted a mini-tutorial on this topic once that explained it well, but I can't remember where he posted it. Maybe he or someone else will come along and post it.