i don't know what's the matter

对,这句话正确.
what's the matter这句话充当了know的宾语.一般情况下宾语从句要用陈述语序,但是what's the matter比较特殊,它在充当宾语从句时句序不可换,它等同于what's up.这句话也可以换成i don't know what's up.