Offended how? That she has yet to ask you and could be assuming your answer is yes or that there might be a possibility she won't be asking nor expecting you to stand with her?
I don't think you should be offended either way and don't just flat out ask her - that might cause her some undue embarrassment or force her to answer a question she's not ready to answer. If she's not planning on you being in the wedding, she may be finding it hard to tell you since she was your maid of honor.
Just remember, this is HER wedding and she's your friend. Offer her assistance whenever she might need it and let her take it from there. You will know soon enough what she is going to need from you - in her time - about her day.
Sorry I'm so blunt. I had a maid of honor and a matron of honor and the two of them argued until it came time to walk down the isle over which one should stand next to me.
I really wanted to kick them both outta the church by that point.