What Can I Do Now?

Since my manager left, I've only been given a single 4 hour shift on Sundays and it's definitely not what I want. I've already changed my availability to show that I'm able to work throughout the week, but still no change. I've spoken to my assistant manager twice that I wanted more shifts, but she just used petty excuses and lies like "not enough hours to share" when my co-worker had at least 12 hours that week, and "not experienced enough to be on the floor," but seriously, how am I able to improve when I'm not even given the chance to? And now she scheduled an appointment to speak with me tomorrow. About what? I really have no clue. It could be a performance evaluation, or an opportunity to fire me. I don't know and it's really bothering me... What can I do now?