Contrast-Induced Nephropathy (CIN): This is a common complication of contrast dye administration, especially in patients with pre-existing renal impairment. The symptoms of feeling unwell, headache, and back pain could be indicative of kidney dysfunction or pain.

Allergic Reaction to Contrast Dye: Although less common, an allergic reaction to the contrast dye could cause a range of symptoms including headache, back pain, and general malaise.
Dehydration: Dehydration can occur due to the diuretic effect of the contrast dye, leading to symptoms such as headache and back pain.
Anxiety or Stress Reaction: The experience of undergoing a CT scan and receiving contrast dye can be stressful, potentially leading to anxiety-related symptoms like feeling unwell or having a headache.

Contrast-Induced Anaphylaxis: Although rare, anaphylaxis is a life-threatening condition that requires immediate recognition and treatment. Symptoms can include severe allergic reactions beyond what is typically seen in a mild allergic response.
Severe Renal Failure: In patients with pre-existing kidney disease, contrast dye can precipitate severe renal failure, which is a medical emergency.
Thyroid Storm (in patients with thyroid disease): Iodine-based contrast dyes can precipitate a thyroid storm in patients with untreated or undertreated hyperthyroidism, which is a life-threatening condition.

Thromboembolic Events: Rarely, the contrast dye could potentially dislodge a thrombus or cause a thromboembolic event, especially in patients with pre-existing vascular disease.
Idiosyncratic Reaction: A rare, unpredictable reaction to the contrast dye that does not fit into the typical categories of allergic or anaphylactic reactions.