She has Heparin induced thrombocytopenia, which can be deduced due to the decrease in platelet count after medication administration and the fact that she was being treated for a massive PE. Heparin is used for immediate anticoagulation of PE and to treat DVT recurrence (FA2020 p436).

Symptomatic HIT (Type 2) is caused by development of IgG antibodies against platelet bound factor 4.

The mechanism of heparin is potentiation of the action of antithrombin, which decreases the action of thrombin and factor Xa and therefore acts as an anticoagulant

Also good to know that only Unfractionated Heparin has the ability to (1) bind to factor Xa and therefore potentiate Anti-coagulation and (2) bind to Anti-thrombin III and Thrombin thus bringing them together and therefore potentiate Anti-coagulation

Fractionated Heparin can only do (1)

Reasoning is due to the difference of size between the heparin molecules and what they can bind to

(see Uworld Qid# 2132)