MIL issue

Brittney
My SO and I bought winter clothes for our LO this weekend because it's finally getting cooler and she had absolutely no warm clothes or pjs. My MIL says, oh I told you you should wait til Black Friday for sales and such. To which I replied that she could not wait that long because it is getting cooler now. So today we bought another sweater and some more pjs and she says it again to my SO. 
I don't know if I'm being extra irritable because it's almost that time or what but that really bugged me. I already told her why we didn't wait and yet she says something about it again. I'm or gonna make my 8 month old wait for warm clothes just to get them cheaper. 
And on a side note, we normally get along great, although more and more is starting to bug me, but I bite my tongue. 
I haven't said anything to her. I am just quietly stewing. Am I overreacting? 
* this is my first post, not sure what topic I put this under or how to do that. So sorry if it's in wrong place.