I've worked in the bank and I had a simular problem. I learnt that being firm with my manager worked rather then being upset and down.

If I were you id go to my manager and tell her my concerns about the situation and also let her know that the only reason you stepped down to part time work is due to the incorrect informatin she gave you. Tell her you have no desire to step down your current position and would like to know how SHE is going to fix the situation. She has a duty to be there for you as your manager. If that fails Id go above her head..even to the department that takes care of the positions ect.

I totally agree with MR - stay in your position for as long as you can...I worked up until 38 weeks, full time in my branch and was thankfull I didnt take maternity leave earlier oh and dont forget...make someone else carry all those dame coin bags