Lump under skin, back of thigh, invisible on MRI. Is this a lipoma? What else could it be?

Hello! I have a hard lump under the skin on the back of my thigh. The lump is about the size of a chickpea, maybe slightly bigger, the skin above it is not discolored. I was told that it could be a lipoma. On an ultrasound it was visible, but when I had an MRI done, it was completely invisible. Does that exclude a lipoma? What else could it be? What kind of tissue is invisible on an MRI? What kind of lump should be removed? Thank you! (I'm female, late 20's, BMI 21)