D-dimer very high

A couple of weeks ago my mother in law had a brutal syncope when getting up to go to the toilet during the night. I (CCU nurse) was informed of this about 10am. Needless to say I sat in my car about 5 min later. BP and pulse were normal at that time. I could not convince her to go to the ER.

2 days later she went to the GP and her lab results showed a d-dimer level of aprox 9000 and she was lightly anemic.

She went to hospital and had a CT Thorax and an ultrasound to rule out PE/DVT. Both came back negative. The following days she had a CT abdomen, also negative. She was also seen by cardiologist and a gastroenterologist. During the week in hospital the d-dimer levels dropped and no cause was found. So she was discharged and told to let her GP do the follow up.

Now she feels very fatigued and labs just came back: D-dimer levels of >10000. The rest of her routine bloods are normal.

What could be causing this?

I am definitely taking her back to hospital in the morning.

thanks