That was really rude what she said to you. Completely un-called for!! Ugh people like her make me very mad!!
Anywho, I forget that I have strech marks, they are there, they will always be there just lighter and I deal with them. It will always be a reminder of how far you have come!
Keep your head up and dont let people like you BF's mother get you down, she must not be comfortable with herself or she wouldnt have said that!