Your mom needs help.

Maybe you should collect your used pads and show them to her in the evening. So she can make sure. Seriously, wtf. (My go to theory is there was a stain on the underwear, which happens even if you change pads or tampons regularly. But what a weird reaction.)

Someone is certainly on the rag.

And why exactly is that any of her business anyway?

They found out she was changing it at home when someone barged in on her while in the bathroom.

Um, I struggle to see how not changing a pad would warrant such a reaction... Maybe... No, still don't get it But seriously, has she lost her mind?

I am guessing the reason for her behaviour could be for (either of) the following reasons: You change so often that she thinks you do it only at home (garbage filled with pads), and/or there are traces of blood on your underwear (believe you don't change it often enough, hence the leakage - you will need the thicker and longer ones if so). Regardless, her behaviour is strange, so I suggest that you have a talk with her.

So Mom's not talking about her kid's blotter pad on her desk at school?

How tf does she even plan to figure out whether or not you change it at school? Stakeout in the bathroom? Ankle monitor?

You should gift her sessions at a shrink for the next gifting occasion. Or just grab whatever you can hold and run far, far away.