Sounds like she is in a very complicated situation for such a little girl. I'm guessing grandma she considers her mom and you her mother. I have no idea if you absence is really a big reason for her issues. Who knows. If she has always really had grandma as her "mom" pry not though but if not that could be a culprit.
Regardless of all that she hasn't been in school long at all! Remember that. She is only 5. I would have grandma reinforce constantly how important studies are on a daily basis. I'd also ask grandma to sit down with her nightly and maybe work on a store brought workbook for a child her age. If nothing else reading together. Maybe she just needs to get excited about school. But I'd for sure get the school district to evaluate her for a learning disability. But she is really little and I wouldn't put all my attention to that.
School is a new big thing when you are 5. She isn't pry even that well acclimated to it! So just get her excited about it I'd recommend.
Also,it never hurts to ask her what is going on in her mind over ice cream. Being just 5 she can still maybe articulate the issue.