I get shaky and feel weak if I don't eat every few hours?

I was in for a complete checkup 6 months ago. I'm 54 and have kept myself in pretty good shape. Since I just went through menopause, and I hadn't had a complete checkup in a while, my doctor ran a lot of blood tests. She checked my cholesterol, my blood sugar, my thyroid, and quite a few other things. All my tests came back great, she even remarked that my cholesterol was better than hers, and she's only 31. Well, she got married and moved away, and I don't have a new doctor yet. The past two weeks, I've noticed that if I don't eat every few hours, I feel weak, shaky and nauseated. I haven't changed my eating patterns, nor gained any weight, and I work out in our apartment's exercise room at least 5 times a week, so I don't know what might be causing this. Any ideas would be appreciated. If it keeps up I will find another doctor, but I'm at loss as to what might be causing this when my test results were so good and I haven't changed my lifestyle at all.