Pelvic girdle pain is normal but nothing to do with small built or not. Your gynae will also say its normal. It's due to the weight of your growing uterus.
I was at 45kg during my first pregnancy and also small built. No, I did not experienced any pelvic pain. I only experience the pain now with my current pregnancy at 21 wks.
Shouldn't feel too painful. If it is then better to see your gynae.