I have what is called a 'sacral downslip' where my left pelvic bone slips down, out of place. This causes my femur to rub on my psoas and groin muscles. That day it was really tight and adding the aggressive position; something popped. The Doc never told me which muscle but the next day it was massively bruised in that area. Over 5-8 years I'd train until I'd get another injury relating to the upslip (typically quad strains), and it took a specialist in another state to get a real diagnosis.
Yoga, an SI Belt, and a few correction exercises keep it at bay. For a permanent fix I can get screws put in there, but there's a chance if its done wrong my days of riding a bike are over so I stick with the routine.