How can I find out what is wrong with my niece?

hi! im traing to find help! im 18 and my niece is 14 and shes been acting so wierd lately. she dosent want to eat and all of a sudden she hides in her room and dosent come out 4 hours. when I ask her whats wrong she sais shes fine and walks away. I dont know much abaut kids! I know I was once one but I just didnt expiriance things like that, she is scared of interacting whit people and baks up from everybody, we have tried concelors but all she dous is sit there and say nothing and if the councelors try to get close to her she baks up! teachers at her scul tell me tat she is suddenly so quite and she dosent hang out wit friends anymore and she keeps away fom people as much as possible! I dont know what to think! I cant really think of something that is affecting her that bad! when she goes visit dad he sais shes the same and dosent say anathing!! she only haves me and I would really like to help! I would just want to know hoW!

please b as helpful as you can! thanks!