Take a test. If you test positive, it is rarely wrong. If you test negative, it may be too soon. Often the first sign of pregnancy is sore nipples returning after a great nursing relationship. That being said, there are many reasons why you could be having sore nipples. Has your baby recently changed how much she nurses? 6 months is spot on for a growth spurt, so if she is nursing a lot more, that could be a reason. My kids also got really distractable between 6-8 months, so we had lots of popping on and off, which caused sore nipples. She could also be teething, and changing the way she latches on. As another sign of possible pregnancy, I got the "heebie jeebie" nail-on-the-chalkboard feeling up and down my spine whenever I nursed my toddler while I was pregnant.
Many signs of early pregnancy are similar to premenstrual signs, so you could also be getting ready to get your period back. I would take a pregnancy test. If that is not the issue, you might do well to contact a La Leche League leader, or a lactation consultant and have a chat about what might be changing in your nursing relationship that is causing whatever "odd" symptoms you are noticing. If that doesn't do the trick, you can always talk to your doctor. (Be aware that doctors usually have extremely minimal training about breastfeeding, so you may be advised to wean your baby when there is no actual reason to do so.)
Best of Luck,
S.