Why would i be put on an antipsychotic for mania rather than a mood stabiliser?

Hi. I am 24 and live in Glasgow,Scotland.I have been hospitalised twice over the last few years with depression.I see my psychiatrist yesterday. I had been experiencing a high and elated mood for about a month before i went to see her. I wrote down how i had been feeling and took it with me to my appointment.She said it sounded alot like a manic episode.She put me on 2mg of Risperidone for the mania.She said i might have Bipolar Disorder but she isn't sure yet. I have to see her again in a months time.Why would i be put on an antipsychotic for mania?Wouldn't i need a mood stabiliser? Thanks