When I used to try autofellatio as a teen, I had reflux. A lot of that is down to diet. It could be that you are intolerant to something, perhaps wheat or spicy food.
In plow pose it would be worse as your stomach is above your mouth. I do it mostly in plow because it is easier for me to get contact. Do your self sucking several hours after eating or better still while completely empty.
As for back pain, yes it happens. If you stretch regularly, the back should get more used to bending. Always take care and perhaps work on stretching your back by other means like in the gym.
You should always take care not to over do it especially early on as you could injure yourself. If you feel pain, back off!
It takes time for most to do this so don’t rush it
