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.
For me normal. If you're worry then go see gynae, they know better than us after all ☺️
Best to see your gynaecologist asap take care!