I'm a PT. It's generally fine to sleep on your stomach. There is no one size fits all for positioning. If you are having issues, then it might be worth seeing a PT to assess and give guidance but you don't need to change things if you are sleeping well and waking up without issue
With or without a pillow? I like to sleep on stomach but can’t use a pillow. Then after 5 mins when I move to my side I have to find the pillow and wrap around my head. Then after 5 mins I’ll go to the other side and have to flip the pillow. Then another 5 mins I’ll go back to stomach, ditch the pillow and the cycle continues.
You literally said your girlfriend forced you to change your sleep style. And you said to your back, not to your side. Conveniently your message is gone now, because you edited it to shape the narrative.