The reason I think that it may be a tension problem is that I've also noticed tiny bumps or loops on the underside of my stitching. (I don't think it's an ironing problem because I'm always very careful to use a dry iron and to press up and down rather than from side to side.
I guess I need to pull out my sewing machine manual to figure out what sort of adjustment I need to make. It's so unusual for my Bernina to have any sort of tension problem that I can't remember whether bumps on the bottom indicate a problem with the thread tension or the bobbin tension.
(And yes, I changed the needle.)