I have only had one Siamese, and he was quite a bit noisier than any other cat that I've had. They tend to be talkers. Usually, cats reserve their meows for communication with the people who are important to them, but it can be hard to know what they're trying to say. It could have something to do with the change in food, although most (not all) cats prefer wet food to dry, although the debate between which is better has supporters and detractors on both sides.
Stress is a common cause but that's not overly helpful since it doesn't help you know what she might be stressed about. You don't say how old she is, but I know that I started adding wet food to my cat's diet after they became older, concerned about their hydration. Older cats sometimes get disoriented, and cry, particularly at night.
One of my cats does that. Relatively silent most of her life, she began yelling at night. I found that leaving some lights on around the house helped a lot; at twenty-four, she might have some trouble seeing in the dark. Still, she yells sometimes, usually at night. Thinking maybe she is lonely after my wife and I have gone to bed, we began leaving the bedroom door open and inviting her to sleep with me. Sometimes she does, but mostly she gets up, goes out into the living room, and yells. Leaving some lights on for her helps a lot, but it doesn't entirely solve the problem.
