It could be improper form, a weak back, or a mix of both.
You should keep your chest and head up when SQUATTING, and avoid leaning forward. Use a weight which allows you to perform proper form and avoid injury.
Your lower back is involved in squatting, so if you have a weak back it could be causing some pain. Push through your legs and heels especially, to avoid putting much strain on your back.
Even if you squat with correct form, your lower back may be slightly sore if you are new to the exercise.
Watch the video below to maximize your squats!
Maximize Your Squat For More STRENGTH, FLEXIBILITY, BALANCE & OVERALL POWER!