How do I keep gaining weight like this? I never used to!! 10 points!?

I am now 22 and I don't eat or act any different physically than I ever have. I can even remember a year ago posting a question criticizing overweight people, saying I just eat whatever and I don't gain weight, so they must be doing something to be overweight. But whereas my weight used to remain steady no matter what I ate, it now seems like I keep slowly gaining weight even though I do excercize every day. For instance I recall in May I weighed 145 and now I weigh 160. Likewise I eat normal food like cereal or oatmeal for breakfast and fish for dinner, and I just eat a normal amount, not a lot. I just have to eat what I can afford to get. So I really don't understand why now all of a sudden I am gaining weight like this, even though I eat normally and excercize regularily. I do have lupus, but I don't know if this would explain it, because this never used to happen. So what could be causing it and is there a safe way to stop it or lose weight? 10 points! thnks.