Has anyone else returned to work and found their job isn't there?

I've returned to work and the person covering my maternity leave is now permanently in the post. Although I was told via email that a new position was going to be made for this person, my exact job was advertised in the newspaper while I was off (for 10 months) which was a surprise. I have tried to not let this worry me and have gone back to work with an open mind - and I've kept in touch with my employer - all seemed ok and I believed it would work out well as this person is a great addition to our team and has different skills from me.

I've been back for two weeks and although I get on well with and respect the person in the job, I feel like I have gone back as this persons assistant. I no longer sit on the committees I used to. Part of this is circumstantial - because I am part time I can't attend some commitees and also the person sits at the desk where I used to sit. However, when I can attend meetings, the person also attends them rather than giving them over to me as a responsibility. The person also tries to tell me how to deal with some situations and when I deal with another staff member the person interjects a lot. I honestly feel like I am a trainee. I have been in the post for 4 years and this person for just 10 months.

Effectively this feels like a job share or less - I am being passed work by this person at their choice. Even though we have made attempts to divide responsibilities, I'm only being passed a small amount of work of a very low level. I'm not sure what the person doing my job is working on, but they are extremely busy whereas I am scratching around to fill 3 days a week at the moment.

My supervisor is on leave and this person is acting manager - so effectively my supervisor, but has not addressed the situation at all. I can't help feeling pushed out. I keep trying to supress these feelings, but last week I felt so down about it I was in the toilets crying. I spend my lunch breaks breastfeeding my baby and it's really difficult just being at work, never mind dealing with these extra issues.

I am trying to stay open minded, but this experience is going to effect whether I return to work full time as at the moment I don't feel like I have the job I left behind, or an equivalent one for that matter.

I want to be able to approach this person, but feel awkward as I can understand what it must be like to own a job, and then have the post holder return. I honestly don't think any malice is intended, and I think this person doesn't have the skills to effectively address the situation which is part of the problem. This leaves it up to me though, and I'm not sure I have the skills either! I'm worried about sounding like I@m complaining. We also don't have private office space and are constantly interrupted making any kind of formal meeting impossible.

Should I leave this a few months until my actual supervisor returns and deal with it then? Has anyone dealt with a similar situation? How would you handle it?