Cat experts?

Hi, my Cat has been really sick the past couple days.
It started with the canned tuna I gave her. She threw up tuna twice. 
Then she just sat in a ball on the counter, and wouldn't eat her food, and wouldn't drink any water. I made sure food and water were fresh, as I always do. She's been gagging since then, this watery sounding gag, but doesn't throw anything up.
(I gave all three of the cats the tuna, add only she threw it up.)
We went out of town for 2 days and my mom came over to feed them Saturday, she said my sick kitty ate a little and was purring when she picked her up.
We got home today, Sunday, and her food was barely touched and she looked sleepy. She's moving around a little more, but hasn't eaten but just a few bites. She's drinking some water now. 
And she's still gagging, and nothing's coming up. It sounds liquidy, but it never comes out.
I'm wondering if anyone has any idea or advice, as we are broke and my husband just startedd a new job and hasn't gotten his first check.