My 3 y/o was treated for croup 10/11/12. cough since then, fever started yesterday. hr went from 112 to at least 140 overnight.?

Call his doctor.: His symptoms may not be related to his history of croup, which should have gone away after treatment. You son may have a cold or flu, or require antibiotic treatment depending on the cause of his symptoms. Try tylenol (acetaminophen) and fluids, check his temperature, and call his doctor today.

Likely unrelated: Depending on when this question was asked, current symptoms are likely not related to october's croup. Most likely a viral pneumonia, but visiting your pediatrician is recommended.