Teething usually starts a few months before a tooth makes its actual appearance. And some babies do get teeth really early--my nephew got his first tooth at 4.5 mos! Usually the bottom center teeth come in first, but it's possible she's getting her first tooth on top. It's so hard when they teethe this young, cause they can't hold a teether, etc. themselves. Try holding a cold teether to her mouth for her & let her gum it for awhile.