Um... to know if she's pregnant you'd have to at least see a few of the following signs:
Morning sickness
Missing period
Fatigue
Frequent urination
Dizziness/Fainting
Sensitive to scents/aromas
Food cravings
Breast tenderness
AND MORE.

That means, I'd say that she'd need at least like 3 of those symptoms to even think she's pregnant. Nipples being tender alone doesn't make me think she's pregnant could be something else, I'm not a doctor. Maybe you bite them too hard during rough sex?