Acute Withdrawal from Alcohol or Benzodiazepines: Given the patient's presentation of jerking arms and legs, severe pain all over, increased muscle rigidity, and a history of appearing homeless, which may suggest a history of substance abuse, acute withdrawal from alcohol or benzodiazepines is a strong consideration. This condition can lead to severe physical symptoms, including seizures, which could explain the jerking movements.

Seizure Disorder: The patient's jerking movements could be indicative of a seizure disorder, which could be primary (e.g., epilepsy) or secondary to another condition (e.g., head injury, infection, or metabolic disorder).
Neuroleptic Malignant Syndrome (NMS): Although less common, NMS is a life-threatening neurological disorder most often caused by an adverse reaction to neuroleptic or antipsychotic medication. It presents with muscle rigidity, fever, and altered mental status, which could align with some of the patient's symptoms.
Serotonin Syndrome: This is a potentially life-threatening medical condition resulting from an excess of serotonin in the central nervous system. It can present with agitation, confusion, rapid heart rate, changes in blood pressure, and increased muscle rigidity, among other symptoms.

Stroke or Transient Ischemic Attack (TIA): Although the patient's presentation might not classically suggest a stroke, it's crucial to consider this diagnosis due to the potential for severe and irreversible consequences if missed. Some strokes, especially those involving the brainstem or posterior circulation, can present with unusual symptoms.
Meningitis or Encephalitis: Infections of the meninges or brain can present with a wide range of symptoms, including severe headache, fever, confusion, and sometimes seizures or abnormal movements. These conditions require prompt diagnosis and treatment.
Hypoglycemia: Low blood sugar can cause a variety of symptoms, including confusion, tremors, and even seizures. Given the patient's disheveled appearance and potential for poor nutrition, hypoglycemia should be considered and quickly ruled out with a glucose check.

Tetanus: Characterized by muscle stiffness and spasms, tetanus is a rare but serious condition in developed countries. The patient's increased muscle rigidity could potentially suggest tetanus, especially if there's a history of a recent wound.
Rabies: Although extremely rare in humans, rabies can cause a range of neurological symptoms, including agitation, confusion, and abnormal movements. Given the patient's altered mental status and jerking movements, rabies could be considered in the differential diagnosis, especially if there's a history of an animal bite.
Dystonic Reaction: This is a rare but potentially life-threatening side effect of certain medications, characterized by involuntary muscle contractions. It could explain some of the patient's symptoms, such as muscle rigidity and abnormal posturing.