Discomfort during vipassana meditation

I have only started meditating for only 2-3 weeks. But now I cannot seem to meditate properly. Whenever I sit for meditation, there are several problems, like increased salivation, feeling of being pressed in the chest and like not getting breath after a few minutes, which to go away I have to take a deep breath. The same just process repeats itself for the whole 20 minutes I sit for meditation. I sit in the burmese position with pillow under my butt, on the bed. I would be glad if someone please helped me.