It would be infinitely sad if your ladyfriend was enduring grimace-worthy pain every time you did it just to be polite. Unless you left out a key part of your question (PS: I am an abusive monster around whom she doesn't feel safe speaking freely) I think let's go ahead and assume that she is actually trustworthy when speaking about her own body and not a liar.
Eliminating also the option that she is just messing with you in some way, because that is weird and why do that, we are left with several possibilities:
1. She is not in pain. What appears to you to be a grimace is, in fact, a rictus of nearly-unbearable ecstasy. On the plus side, nearly-unbearable ecstasy caused by your penis, on the minus, rictus.
2. Or, she is not in pain, nor in throes of ecstasy, that is just a face that she makes sometimes for a reason unknowable to us. Plus: pain-free. Minus: the unsolvable facial mystery reminds us that we will never truly know the mind of another human being, even in the moment of our most intimate sharing, we are separated by an unbridgeable gulf. Bummer.
3. Perhaps she is in pain after all, but she endures it because she likes the pain. Pain gets her off. Plus: she gets off. Minus: your lovemaking hurts the one you love. Plus part two: but it is a hurt that she wants and likes and enjoys.
4. She is in pain, but not because of your penis. Maybe she has been getting those weird toe cramps, or her leg muscles are achy, or has a charley horse, or her hair is caught on something. I would hope that she would still let you know what's up and rearrange to be more comfortable, but again, unless she actually scared of you for some reason (which, BIG MINUS), the onus is on her to get herself sorted.
Or perhaps it's something else altogether. At this point though, it seems to me you have done all you can do, other than just making sure the sexual lines of communication are open between you guys.
Ladies, gentlemen, how many times do I have to say it? Talk to your sex partners! It is the best way to ensure your sex is how you like it, and that your boyfriend isn't writing into me concerned about your sex rictus.
Can you really regrow your foreskin if you are circumcised? How?
Well, kinda. There is a surgical skin graft method where they take ball skin and sew it to your dick. The more popular way involves tugging the remaining sheath skin until it covers the penis. According to the internet, there are basically tons of methods and devices for this, ranging from just yanking it with your fingers to tape to buying a fancy machine.
Also, good to know, the foreskin people get very shirty if you imply that one is stretching the skin to cover: apparently you are actually re-growing new tissue kinda/sorta, like how people with lip plates or earring expanders do. Check out an organization called NORM for helpful hints and information, or also google is your friend.
Either way, I can't say that the process looks particularly comfortable or pleasant, but apparently it works. You too can achieve that sleek uncut look through sheer dedication to daily sheath tugging. Maybe he's born with it/Maybe it's the RECAP foreskin restoration machine, etc.