I probably know the answer but you never know.
When everyone eats the wedding cake at Pedro and Rosaura's wedding, they cry first and then you see everyone puking. Do we just assume it's from the agony and stress of watching your bf marry your sister (that Tita emulated through the cake?) We never saw Tita vomit, though it's obvious she was in agony.
I always assumed Nacha had put something on the cake, that's why she kept insisting Tita to go to bed. Like at first the guests are hit by Tita's magic and they're overwhelmed by sadness, then they become sick because of whatever Nacha put in the cake. The fact that Tita finds Nacha dead but seems to be sleeping and has a picture in her hand, only reassures me that maybe she spoiled the cake on purpose, then died peacefully. I don't remember Tita eating any of the cake, I'd have to watch the movie again though.
sharethe movie has a lot of tita's bodily fluids getting into food and people being affected by it. her tears got into the cake and it made everyone mourn their lost loves...
shareTita's tears that fell into the cake while she mixed it, was what caused the wedding guests to first mourn their lost loves, and then get violently sick. Tita never vomits, but I can imagine how her stomach must have felt with all that hurt and terrible sadness held in.
Nacha I think died from grief in a way, clutching an old photo [and she was very old too].
It wasn't so much her teardrops that tainted the cake, it was her spirit. When she was happy the food was good and when she was sad the food was bad. Everything she made had a piece of her soul in it.
I think that happens in real, non-magical life, too. When I am cooking something for an event with people I love, it turns out great and we all enjoy sharing it. If I am sad or distracted because I am dreading a meal and the people we'll be eating with, it's not so delicious.