Check your initial posture. Your weight should be slightly forward. Your spine should be straight, don't drop your head or curl your spine. Make sure you're not leaning to one side or the other.
If you're doing zazen type meditation and sitting cross-legged, you can put a pillow under yourself to lift the area under your spine to bring your weight forward. This should also occur if you're sitting in the seiza posture. You can support yourself by putting a pillow between your legs while sitting seiza.
A short meditation doesn't give you enough time to find errors in posture. Only when you attempt longer sessions will you feel the differences.
Good luck with your practice.