It doesn't make sense in english to say something is "just now". It would only make sense if the sentence was "It got warm just now", or "It is warm right now". Right and just are not one in the same. They can mean the same thing depending on the context, but it does not make sense in this context. American English
In British English people do say it either way; "it's just now warm" and "it's warm just now" both work but have slightly different meanings. You'd say the first one if the thing had only just happened very recently, and the 2nd one indicates that the thing is happening at this present moment but doesn't tell you about the past or future.