I Have An Extremely Hard Time Gaining Weight. I Was

I have an extremely hard time gaining weight. I was a size 7, until college. I was very physically active in college and ended up losing about 25 pounds in three months. I was 137. However, my weight loss issues did not stop there. Since then, over the past two years, I have steadily lost weight constantly throughout the time period. I'm probably in the 90s as far as weight goes at this time. Size 0 jeans fall off of me sometimes. I can sometimes put on 5 pounds or so, but seem to drop it with a week. I am a heavy daily cigarette smoke and have been for almost 3 years. Other important details.. I'm a 21 yr old female and 5 foot 4. I recently bought some women's vitamins and have been taking then for two weeks without realizing they boost metabolism and have lost weight a little more rapidly since taking them, it seems. I stopped taking them. I can hardly gain weight. I definitely can't keep it on, but continue to lose it. If vitamins didn't help, what can I do to get back to a healthy weight and stay that way? I eat healthy. But always light. I can never eat a lot. Also, are there any medical conditions that could perhaps be causing my sudden inability to gain weight??