In English when someone decides to reconsider a decision they say "Now I am thinking" It is used as often if not more often than " I am thinking now". Strengthen exercise, Gerund.
I don't get it think is not supposed to be Νομίζω? Does it change of form at the Medio passive?
Νομίζω is "to think" in the sense of "have an opinion", as in "I think you're right" or "I think that she is pretty" (= it is my opinion that you are right or she is pretty).
Σκέφτομαι is "to think" in the sense of "be in thought, ponder, cogitate, work things through in your mind" -- for example, if you want to think about a question before responding, you'd be σκέφτομαι-ing, not νομίζω-ing.
I wondered why the exclamation mark. We're always thinking. The comments here make it clear that we use the expression when we are considering something, or trying to answer a question. Actually, I wouldn't say I'm thinking about it, but "I'll have to think about it". Sometimes meanings are "lost in translation", but sometimes we learn how other people "think".