It depends on how much she is bleeding.
If she is just spotting, she's probably just fine, and it's likely just due to some small abrasion from friction, which happens. That can be avoided usually by being sure to use plenty of extra lubricant with your condoms as needed, and sometimes applied more than once during intercourse.
If she is not spotting, but is bleeding profusely (as she might be with a very heavy period or heavier than that), she needs to go to her doctor or the ER.
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