A 10 y.o. boy was ill with angina 2 weeks ago, has complaints of joint pain and stiffness of his left knee and right elbow. There was fever (38,5°) and ankle disfunction, enlargement of cardiac dullness by 2 cm, tachycardia, weakness of the 1st sound, gallop rhythm, weak systolic murmur near apex. What diagnosis corresponds with such symptoms?