It could be just that she's spent a lot of time with you. Though I do think it's rude for anyone to ask. 1) if you're actually not pregnant (then twisting the knife if you're having infertility issues) or 2) if you are and aren't ready to tell. I knew my SIL was pregnant before she announced (you could tell by looking at her - she wasn't even that far along), but it's simply polite and respectful to not say anything and let them announce when they are ready to and in the way they want to.
Even if your mom suspects, she may not have said anything so you can tell her. Even if she's upset, just be honest and say you wanted to wait and make sure things were okay first (or whatever reason is). Regardless of your reason, it's your and your husband's decision to make, and it's not wrong. If she can't accept it, she'll eventually come to terms with it.
My mom knew with both right away, but everyone else didn't know until a little later. My dad was upset that she knew so long ago and we didn't tell him, but eventually understood our reasoning and respected that.