Baby grimacing when given pacifier??


My 5 week old just recently started last week grimacing and then spitting out her pacifier. The look on her face almost says she's in pain. I'll try one or two more times after she spits it out and she'll take it without grimacing. I've looked in her mouth and I don't see thrush or anything. Anybody else experience this? I asked the pediatrician yesterday and he said he had no idea why she would be doing it.