I feel like my body is going into starvation mode. What should I do?

I'm a 19 year old college student. The majority of food in the cafeteria is unhealthy, so I avoid it when possible.

Here's what I eat:
Breakfast: A fiber one bar
Lunch: Iceberg salad with cucumber, cheese , and little ranch and two cups of water
Dinner: Same as lunch

However, I'm gaining weight and I don't know why!! I'm eating healthy.

What am I doing wrong here?

I also go to the gym and do a lot of walking.
I feel like my body is going into starvation mode. What should I do?
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