Did you cleanse the tourmaline crystal before you started using it? It could be that it was already heavily charged and it makes you feel like this.
But also, I have not used tourmaline but since it is a black stone then I think that it absorbs energy, specially negative as this is a property of black crystals. Do you feel like this sadness is something that you are releasing somehow? Or do you think this stone is hurting you?
I think black crystals can more easily absorb these emotions so it is possible that it is bringing up buried energy to be released so you are experiencing the emotions right now.
If you feel like the stone is hurting you, then you could try cleansing it first and then using it again and see how it affects you.