low blood sugar undeweight or hypoglycemia?

Righto, here's the thing. About three and a half, four weeks ago I noticed my blood sugar dropping a couple of times -- shakiness, the urge to eat, nothing too drastic. I went to the doctors and they took some blood to test for the usual culprits like hyperthyroidism (I have a family history of it), anemia, diabetes. After a week of feeling tired, needing to eat constantly but not feeling like I was raising my energy levels, I went back to the doctors. All my tests were clear, which is why I hadn't heard from them, and my doctor said that I seemed a little underweight and that was perhaps causing my malaise (I'm 21 years old, about 5ft 3, weigh about 47kg). I admit that over the last couple of months, I've only really been eating breakfast (toast mainly) and dinner with a small snack in between.

So for the past six days I've been eating like the proverbial horse. I know that putting on weight is going to take time so I won't feel great immediately. However, at the moment I still feel very tired, need to eat quite regularly, concentration's pretty shoddy, and have moments of low blood sugar still. I also have started waking up at about 7/8am feeling 'odd' and not being able to sleep until I have a snack (I did used to eat my main meal much later at night, I'll just add). I'm quite the healthy eater, eating mostly rice, pasta, vegetables, loads of fruit, porridge, (added nuts and seeds to my diet since been told I need to put on weight -- I'm vegetarian). Anyway, after all this my real question is would these feelings be attributed completely to being underweight and will get better as I gain weight, or am I possibly hypoglycemic? I really am quite lethargic throughout the day, but that could be that because I'm feeling drained, I'm not doing as much moving around as I should. Vicious circles everywhere.

OK, I ramble too much, really the main question is whether moments of low blood sugar crops up in underweight individuals, or whether it's actually hypoglycemia that I might have. Thanks in advance.