A 43 y.o. woman complains of shooting heart pain, dyspnea, irregularities in the heart activity, progressive fatigue during 3 weeks. She had acute respiratory disease a month ago. On examination: AP- 120/80 mm Hg, heart rate 98 bpm, heart boarders 1,5 cm left side, sounds are muffled, soft systolic murmur at apex and Botkin's area; sporadic extrasystoles. Liver isn't palpated, there are no edema. Blood test: WBC- 6.7 x 109/l, sedimentation rate- 21 mm/hour. What is the most probable diagnosis?