Vestibular or neurological side effects of antibiotics: The patient's symptoms of tingling, ataxia, lightheadedness, dizziness, and nausea could be related to the recent change in antibiotics from Bactrim to cephalexin. Some antibiotics are known to cause neurological side effects, and the timing of these symptoms in relation to the antibiotic switch makes this a plausible explanation.

Vitamin B12 deficiency: Although less common in young adults, a deficiency in vitamin B12 can cause neurological symptoms such as tingling in the extremities and ataxia. This could be considered, especially if the patient has a history of dietary restrictions or malabsorption.
Electrolyte imbalance: The use of antibiotics can sometimes lead to changes in electrolyte levels, which might cause symptoms like lightheadedness, dizziness, and nausea. Checking electrolyte levels could help determine if this is a contributing factor.
Dehydration: Not finishing the full course of antibiotics and then experiencing side effects from a new antibiotic could lead to dehydration, especially if the patient is experiencing nausea. Dehydration can cause or exacerbate symptoms like dizziness and lightheadedness.

Multiple sclerosis (MS): Although MS is less common in this age group, its presentation can be highly variable, including ataxia and tingling in the extremities. Missing an early diagnosis of MS could significantly impact the patient's long-term prognosis.
Guillain-Barré Syndrome (GBS): This is a rare autoimmune disorder that can occur after an infection, characterized by ascending paralysis. The patient's symptoms of tingling and ataxia, although not typical for GBS, warrant consideration of this diagnosis due to its potential severity.
Stroke or transient ischemic attack (TIA): Although extremely rare in an 18-year-old, a stroke or TIA could present with sudden onset of neurological symptoms like ataxia and dizziness. Given the potential for severe outcomes, this must be considered and ruled out.

Neurosyphilis: In the absence of other explanations, and considering the patient's recent history of a UTI (which might suggest sexual activity), neurosyphilis could be considered, although it is rare and typically presents with more specific neurological deficits.
Toxins or heavy metal poisoning: Exposure to certain toxins or heavy metals can cause a range of neurological symptoms. This would be an unlikely diagnosis without a specific history of exposure but should be considered if other diagnoses are ruled out and the patient's condition worsens or does not improve.