What Causes Pain In Ribcage On Bending Down?

When bending over, it feels like something gets stuck under my bottom right ribcage. It is painful, and I have to stand upright and stretch to make it go away. It does go away, gradually. What could it be? I am a 48 year old female and have never experienced this before until recently. It has happened twice within the last week, both times at the gym, when lying on a mat, pulling my leg towards me to stretch my hip. I am 5' 5" and weigh 123 lbs. I am active and work out 3-4 times weekly. Thanks