R..
I wouldn't stress too much over it. If she won't nurse, then go ahead and supplement with formula. Yes, breast milk is the best nutrition she can possibly get... BUT if you are still nursing her when possible, it's not going to hurt her to get a little formula too. Thousands of healthy babies have thrived on formula alone.
That said... I know that around that same age my DD went on a nursing strike that lasted about 1 1/2 months. Turned out she was cutting 8 teeth all at once, and the sucking was uncomfortable for her. After those teeth finally cut, she went back to nursing full-on again, and I just weaned her this month. (she's 18 months now.)
You might also try giving her a small toy that she can fidget with while she's nursing. That way her hands and mind can be preoccupied and she doesn't get bored with nursing. If possible, maybe you can start nursing her in the bedroom or somewhere with less distractions.