Pretty normal response. The bleeding from the tear clears toward the surface. The worse the tear, the more blood needs to clear out. Don't sweat it.
BTW, Ultrasound is pretty useless. If you can warm-up actively, do so. Also, just in case, dynamic flexibility exercises have been found to be superior in restoring functional flexibility to the hip with a lower recurrence of re-injury of the hamstrings compared to static stretching.
Bill