Yes you were raped, that is not your fault, at 13 whatever the scenario with your teacher means you were raped.
You won't be able to hide it forever so you need to tell a grown up like your therapist.
I'd like to say something comforting like your mother may surprise you and show love at support but your description of how she reacted cause she thought you were fat explains, in part, why you need a therapist.
You need pre natal care to make sure you and the twins are healthy and its possible that you won't go to full term with twins and you all will need specialist care.
You need to tell your therapist as soon as possible to get you the support you need and to work through your options. You won't feel so scared once you tell someone.