Pelvis hurts when I pick up heavy bags

So I'm packing to go on a flight home. I have a duffel bag and my usual school backpack. They're both around 20 pounds(9 kilo) because I packed a crap ton. I put the duffel bag on (it has shoulder straps) and immediately my pelvis feels a sharp pain.

For reference, I'm like 16 years old and very fit. It was more on my left side and I was really confused. I told my aunt and mom, ain't said it was odd, mom was concerned but said I'm fine. (She'll check the bag for me instead of both being carry-on so I don't have to carry them). Just wondering what this could be from???