Follow up, reading back on your mothers valve replacement.
Replacement to me means she may have a mechanical valve, if so I understand that requires the use of a blood anticoagulant. If she had a biological valve an anticoagulant may not be needed. So, please add this observation to my comment about no relationship between the valve work and a clot.
Jerry
Sorry to read you mother had a complication, but I'm not sure it is related to the mitral valve replacement. Did you say she had the surgery two years ago?
Blood clots can be caused by heart irregularities, such as atrial fibrillation (I have that) and atrial flutter. There are other causes which I can not list.
I'd suggest that she talk with her cardiologist about ongoing use of warfarin just to guard against future clots, clots are very dangerous and are to be avoided if at all possible. I take and tolerate well 5 mg of Warfarin daily.