exercise dvd ?

So I recently just started doing this exercise DVD to try and lose a little bit of weight and to keep fit since I don't really go to any clubs or anything. Apart from walking to and from school and around school it's all the exercise I really do. There are 12 rounds of the DVD and they each last 3 mins.I got to 5 yesterday before I thought I should have a break. But when I went to check my levels they happened to drop and I had to quickly get it back up again. I'm away to do the exercise DVD again because it's recommended to doing it 4 times a week. My levels were at 7.0 about 20 mins ago when I checked them and I decided to eat a banana so that it didn't drop while I exercised. But it's not a really good idea to eat right before you exercise because you can get cramps so I'm just waiting 30 mins or so. It's really frustrating because I didn't even realise I was having a hypo after, I thought it was just the exhaustion from the exercise. I'm not really sure to go about this. So I think I might just check my levels after 2 rounds and see what it's at. Does anyone have any tips so that it doesn't drop while I exercise? UPDATE I just checked it again and it's at 4.6?!