How to deal with my dad and the girl I love?

Iv always been really close to this girl and my dad had no problem with her before but over 2 years ago he starts to think that she is a bad person for some reason. I'm still really close and I have feelings for her and she might have feelings for her. Now I think my dad did something to my phone where I can't text her and everytime I go out he's always asking me where I'm going and who I'm going with. When I tell the truth and say I'm going to a store to buy something he doesn't believe me. Her birthday is coming up and iv been wanting to buy her a special gift so I told him I was going out and he knows when her birthday is shoe says that I better not be buying anything for a girl I know. She is my closest friend and she thinks I'm sweet kind, caring guy which makes me want to make our relationship closer. But it's annoying that my dad is trying to destroy my friendship with this girl. What can I do.