My doctor told me to diet and exercise because i am obese. so i started today with a personal trainer a low impact routine with some weight. the thing is i am really in big pain in my body. i exercised before but never felt like this. is this normal?

You should rest for a couple days. When the pain is subsided, you resume the exercise with a less strenuous program and you need to discuss with your trainer. If the pain does not improve with stopping the exercise, your unusual pain may not be due to the exercise or you may have some injury during the exercise then you should see your doctor for a thorough evaluation.

No pain, no gain. Not sure what exercises you did before, but the trainer is going to make you burn calories to assist in your quest. It is quite normal to be sore when you use muscles that haven't been used in a while. Keep at it and you will reach your goal.