As the patient presents with clinical signs and symptoms of DVT, high pretest probability of DVT (high D dimer) but the Wells score was only 3 (clinical symptoms), the venous ultrasound should be performed to rule out DVT.
The venous ultrasound revealed an isolated distal DVT. In addition, the pulmonary ultrasound should be considered if there is high risk pulmonary embolism although not present in this case.
The duration of anticoagulant depends on the risk of recurrence. High risk the patient should be treated with anticoagulation for 3 months where as low risk can be either US surveillance or anticoagulation for 4-6 weeks. (Ref: ESC 2018)
I would perform lab work up for cancer, predisposing diseases eg IBD or thrombophilia for risk assessment.