The word that you wrote on the question is odd in English. Evem American English adapted many foreign languages. "ima" means now in Japanese. I assumed that you have written it wrong, the correct writting is I'am. It is the short form for I am.

Hello Jotape, "I'ma" is a slang and means : I am going to or I am about " I'ma go ta da movies" = I am going to go to the movies". While "I'm " * I am sure you know it* is a contraction of " I am". However sometimes it is used instead of "i've" or "i have" in present perfect ,sometimes called the "perfective I'm" as in : " I'm forgot to do it" meaning " I have forgotten to do it". It is not commonly used and has been noted in the Chesapeake Bay area, particularly among older speakers. it has been also found to be prevalent in the speech of the Lumbee Indians of southeastern North Carolina.