how t stop baby biting her lips

Oh ! dont worry nothing serious like you frighten as she is just 3 months no matter of bleeding of blood,first find out why she does this due to thirst or hungry,or else it is her play and what my expierence say is it is just her activity that gives her pleasure and if she continues it more than a week then apply something that tastes bitter and no harm in taking it inside. Then automatically she stops doing so. All the best.

She's probably doing that too soothe herself or as a cue that she's hungry perhaps? Trial and error to figure out what it means....I wouldn't worry too much unless she's drawing blood or her lips are sore from the biting.