To be honest, OP, even if she did feed your DD formula, it doesn't mean you will start to use formula any earlier than you would have, anyway. It's not like your DD will suddenly get a taste for formula, after one bottle, and go off breastfeeding altogether.
I think it's unlikely she did what you suspect, but even if she did, I can't see there's any harm done. (It's true that there could have been if your DD had some kind of allergy to the formula used, but you'd know that by now.)
Remember, even when your DD is completely weaned from the breast, you still get to say how often she can stay there, and from what age, and for how long. It's not a slippery slope to you being forced to do it.
And what if she comes out with "ha! I gave her formula, and she was fine, so now I can babysit all the time!" Then, you would be perfectly within your rights to say that, as she has shown she does not respect your wishes, you are not happy for her to babysit. So, she would just be shooting herself in the foot.
Having said all that, I am always very suspicious of my MIL's motives, as she is certainly one to play games. (Think changing her will to try to get me and DH to split up.) So I appreciate that this may be the case with you. But you just don't have enough evidence on this occasion that she did what you suspect, so you could just come across as unreasonable and unhinged if you try to make something of it.