Why is my 6-month old waking up at night, not able to be consoled?

She usually sleeps pretty well, and if she cries, usually a pacifier does the trick. The last couple of nights, she is so upset, we have to rock her back to sleep, and we NEVER do that. She is formula-fed, but nothing has really changed in her diet. She may be teething, but she doesn't seem that bothered during the day... just at night?