If she's asking questions, you should tell her the truth. My kids are 3 and 4 and they understand all about where the food on their plate comes from, be it the veggies from the garden or meat from the coop or local farmer. In fact, they don't even use the terms "pork and beef" but rather say "pass the pig or cow." Kids are resilient and curious, and I find I tried to shelter them way more in the begining than was warranted.
My daughter was 3 the first time we slaughtered the birds, and I kept her from it. I then dreaded having the talk about where they went and what that meant (as she LOVED being with the flock). But I found I was projecting my own issues. She was absolutely fine with it after the initial curiosity. She asked where they went ("we turned them into food") and then she was all like "we EAT them?!?" and I said yes. She processed this for awhile and then said "okay, can I go play now?"
Now my children are present at the processing of our ducks, turkeys, chickens if they want to be (and both have always wanted to be) and they have a complete and full understanding of their food. Perhaps that is easier than being shocked into the knowledge when they're older, I don't know. I know I have a very distict and jolting memory of when I first figured it out and I was much older and very upset by it in a way my little ones do not seem to be.
The only time my now 4 year old daughter has cried about this issue is when she saw the small clip of the caged birds in a factory farm while we were watching The Natural History of a Chicken.