Could be a number of things. I know that is how I feel sometimes if I don't have something to eat a little before my workout. Are you kinda light headed or experiencing anything else?
Low blood sugar is common after a very taxing workout. Drink a shake or smoothie, that helps me. Dont forget to eat though soon after to avoid a sugar crash.
Drink lots of water. You may be dehydrated before you even pick up the weights. That will only worsen during your workout.
I weigh 165 and drink about 100 ounces of water a day, closer to 140 when I do heavy cardio. Sounds high to many, but it is what my body needs.
Find out what your body needs. A good rule of thumb is half your body weight in water (ounces). So if you are 100 pounds, drink 50 ounces of water a day at the minimum.
I went through a stupid phase last year and tried to eat as little as possible, sometimes nothing at all, and decided to go work out one day. I did some of my weightlifting, but by the time I was almost finished I felt so nausiated that I just wanted to sit for a while before I could get up to walk to my place. No one 'put me in my place' until after I almost passed out on a tredmill one day.