What Causes Skin Tags On Body?

Yes, I have several hanging moles or skin tags on my neck and also one under my armpit. I had a couple removed by a dermatologist several months ago that were irritated due to being rubbed by my bra. He looked at the others and said they were nothing to worry about. I believed he said they were caused by a virus or something to that effect. I have a hanging one in my armpit that has become irritated and painful. I don't know if I hit it while shaving, but it is possible. It has been painful for about a week and yesterday looked red, today it is not as painful and the natural color has returned. It look like it is drying up. I have had some that became irritated on my neck and then fell off. Do you think this is the case with this one and is this a normal ocurrence become they fall off.