Hi scotsmum
As they are going through restructuring, I get the feeling they are probably just not sure what to do with you, they are used to you not being there, and they aren't sure how its going to work with your part time hours.
Going back to work is such a weird time, and yes things are often very different when you get back.
Not knowing anyone involved, I would give them the benefit of the doubt for now, and assume they just haven't thought much about you. I wouldn't assume they are out to get you, a few weeks is a very short time.
Could you decide on one of the major projects, the one that you could deliver most value to, and insist on being involved? Once you are involved in something, the part-time thing will raise its head "how can you do this if you are only in 3 days a week" etc and you'll be able to allay their concerns with whatever you plan to do ("I'll be picking up emails once a day") or whatever. (Not that I'm recommending you do this because personally I think its outrageous when companies pay 3 days and get more out of people, usually women, but that's another thread :) But you did say you were planning to work a bit extra.) Once you've been involved in something, they will realise you are back, you are useful, and you'll start to be included once again. Hopefully 
I have a colleage who doesn't work Fridays, and she NEVER mentions it in meetings, she just says "I'm not around at that time" if people suggest Friday calls or meetings. It does work well with clients - no-one is 100% available all the time. It gives the impression of being busy and sought after, rather than - shock horror - having other commitments than work. Might be worth considering?
Good luck with it and I hope things improve.