Possibility: Maybe it's too much...

Lick her first - go easy - avoid the nipples or she'll tighten up. Once she can take your tongue on her clit and if she seems lubed then lick one finger and gently slide it in her. Go easy - work that finger around. Make sure it's lubed. Take your time. If she gets really wet then sneak a lick on your palm, transfer the lick to your head, and try to slide it in.