Melissa.. in my experience.. not that this IS the reason for you, but,
Whenever I have felt light headed, while doing workouts, usually my heart rate has gone up too high (when doing cardio). Believe it or not weight lifting does cause your heart rate to go up quite a bit more than you'd think. Perhaps it is that.
Do you have anything you can sit on when doing your weights? I have an exercise ball and usually when doing my arm weights I sit on the ball. Maybe it might help. If you have a heart rate monitor just have a look at it every now and then and see what it says. My dizziness usually came on if my HR went up to around 150 (my 'heart rate zone' chart indicates that's about 20 less than my max. You can find this info online too).
You are doing so awesome with your exercise. Try hard not to compare yourself to the others if they are doing more. Just try to beat YOUR personal best and you'll see progress for sure.
My recommendation is definitely up your workouts in some way. It may be weights or it may be different type of cardio activity. You may also look at what you are eating. Are you eating processed foods? Lots of sodium? Are you drinking lots of water? There are so many things that can affect the scale that it isn't really reliable from day to day.
It was a really hard ime for me but I changed my eating plan and slowly I got things back on track. Also I am happy you joined the exercise thread.
