A 24-year-old woman had fever (38°C) with sore throat, runny nose, and headache 2 weeks ago, which improved with treatment. For the past 2 days, she has experienced chest tightness and shortness of breath. Examination reveals: heart rate 100 bpm, regular rhythm, grade 2/6 systolic blowing murmur at the apex. ECG shows ST-T changes in all leads. Lab results: WBC 7.2×10⁹/L, ESR 25mm/h, elevated CPK and CPK-MB. The most likely diagnosis is: