In the beginning most of the yoga practitioners have some obstacles on their way, for example: pain,heavy breath,headache, or some others. but this is very common. Patanjali also mention his classic book on Raja Yoga that when you wanna practice the holy yoga, then so many obstacles will come in your way to distract you, but this is just like a exam, that can you go on practice or you and your mind will fluctuate and then your concentration breaks, and then the flow of practice also break.If you can pass this exam then you can enjoy your practice for long time without distractions, and if not then your practice always will fluctuating.
   Pain, uncomfort, heavy breath, lack of concentration these are common things in the beginning of practice, but go on practice practice, all will come. You can feel the changes, just need "Abhayasa" (uninterrupted practice).
Enjoy your practice and have a peace around you always.
Hari Om !!

Namaste,
 Manu