passing out at gym

I was recently re-released to return to work out by my MD's. Upon that, I scheduled three paid sessions with a trainer at the gym I belong to. Last week, we made it through 2 exercises before I felt dizzy, started sweating and felt like I was going to pass out. The trainers rushed to my aid, provided cold comresses and gave me grape juice. I should note that I had gotten there earlier than the trainer so i warmed up with 30 minutes on the treadmill - walking. The grape juice helped out and we figured it was because I didn't eat before I went to the gym. I wanted to not get sick on my first session. I worked out on the treadmill between last friday and today without any problems. So today, I ate lunch then napped for 30 minutes prior to leaving for my workout. (it was my day off - I go to school full time and I work full time). I woke up dizzy (hx: I've been tested for Meniere's and Migraines since Feb along with suggestion of getting a fasting blood glucose test in July). I laid back down for 10 minutes. Went to the gym. Successfully walked on the treadmill for 10 minutes and then we got started with upper extremity strength training. My sessions are 30 minutes and I think I made it 20 before I was asking to lay down. And not the type of lay down- I want to quit; the type that my trainer recognized another episode coming on and went and got the yoga mat. Before I knew it my feet were propped up and I was on the ground. When I came home, I passed out on my sofa and woke up 3 hours later.

Has this happened to anyone else when they've begun an exercise routine? I joined to gym to exercise on the basis that walking the dog was no longer working, I was tired of buying clothes up in sizes every month and at the suggestion of my doctors. So far, I've been to the MD more than I did before I began to exercise.
this just doesn't seem normal to me.