My advice: if the pain is really bad no matter the position, she needs to see a gynecologist ASAP. Once upon a time I had similar pain and was given urinary pain medication (available over the counter - AZO). Took the medication, the pain dulled, so it was assumed that I had a mild bladder infection. Antibiotics helped.
If the pain is probably more a result of not enough lubrication or arousal, spend more time on foreplay. Give her an orgasm before you have penetrative sex and her muscles will automatically be more relaxed. And always have lube on hand in case you need it.
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