Mean mil

So Mother in law came to visit with her sister and niece, she's expecting her first baby on July and wanted to meet our daughter and see if I had any tips for her. If I'd had prior notice if have probably wrote a list, at short notice all I could come up with was "Pee in the bath" 
Anyway they were talking away about anything and everything when my MIL suddenly comes out with, "well I don't think she [my daughter] was an accident" and looked at me so horribly! 
For a start we never once claimed the pregnancy was an accident, but I now get the impression that she thinks I got pregnant to trap her son! 
We've been happily together for just over 3 years, yeh it may not seem that long but we know we are meant to be. Our daughter, though not 100% planned (what baby is!)  was also not unplanned, we'd spoken about having a baby, and weren't using any protection. My partner didn't hear her say it and I haven't told him cos I know he'd be pissed with her. I'm so upset that she would think that of me, I'm starting to see a new side to her and I don't like it