Here's some advice I heard on here a while ago, which also helped me a lot in my own life (I'm also a 21F who gained weight during a relationship):

How many meals do you two eat together? What does your own diet consist of? Do you snack a lot? eat 3 meals a day, a lot of meat, less vegetables or anything of the like? eat out once or more a week? Drink a lot of soda or beer or wine? Keep snack foods in the house? Eat more processed foods rather than fresh veggies?

Guys tend to eat that way, and girls eat a lot less (and in my experience, quite healthier) when left to their own devices. I noticed in my own relationship, no matter what I did, I couldn't lose weight. Then I read on here, that girls gain weight in relationships, because they subconsciously eat as much as their SO's during meals, and eat what their SO's eat, which is more calorie-laden and often has more red meats. They subconsciously snack as much as their BF's, and drink more calories via beer, wine juice, eat more dessert more often, eat out more often, etc. as well as eat more often through out the day. You may be enabling this without realizing, though I'm not blaming you. Girls just have to be more conscious of what they eat, especially since you mentioned you are a tall skinny dude.

What I did - and what caused me to almost IMMEDIATELY lose ten lbs (and I'm very small, 5'3) was to stop eating meals with my boyfriend. Absolutely no meals.

I used to eat steaks and burgers with him, breads and heavy soups, 2-3 times a day. And because we spent a lot of time together, I often passed up working out. Now, I eat only one "real" meal a day and have small snacks and coffee in between (remember I'm small though), and I average probably less than 1200 calories a day on average.

Just another angle to look at, it could be a really easy, non confrontational way to help, by evaluating if her eating habits are more similar to yours than they used to be.