Similarly, if your partner is pregnant, nursing, or on their period, their nipples will be tender.
You can increase sensation as you move along and they become more aroused.
After a while, you can give your mouth a break and go back to gently circling and stroking their nipples—they might enjoy the varying pressure.
Luckily, there's a very simple answer to this question: just ask your partner what they like! Ask her to take her bra off.