This is indeed a very unpleasant situation in which you find yourself at this time, especially in the light of the fact that you have, if I look at the posting of your question, now just returned to work.
Firstly, the fact that you were away on maternity leave may not in any way be taken into account when the organisation makes decisions relating to the issues of reorganisation that you mentioned in your question. It is essential to highlight that the Constitution, 1996 makes it very clear that a woman may not be discriminated against on the basis of her pregnancy and being away on maternity leave. And this therefore means that the company cannot simply advise you of a situation shortly before you return to work, more especially when this decision seems to have been taken some time ago. This course of action can only be taken after proper discussion and negotiation – and certainly not a few days before you are due to return to work.
I am not aware of the industry in which you are working, but if there is any union involved I would most certainly, and as quickly as possible, discuss this matter with the union representative who will be able to take up cudgels on your behalf.
If there is no union involved in this matter, I would then suggest that you should as quickly as possible consult with a labour lawyer who will be able to advise you on your next steps and, where necessary, ensure that the company does not take any steps which would and could affect you in the immediate short-term.
This is very broad sweep through your question – please do feel free to submit another question if I can be of any further assistance or if you require any further information.
Good luck – I hope that this matter is settled satisfactorily as soon as possible!