<human>: Summarize the given text in a few sentences.

I'll keep this brief. My roommate, we'll call her Skye, and I lived together previously at school with 3 other people for 2 years. We were both accepted into the same graduate program at a different school and decided to live together again. The lease we signed has both of our names on it and we split the rent (she pays slightly more for a larger room but that was her idea).

Our lease was supposed to go from August 2015-July 2016. This is where the problem arises. She has access to another apartment through family in the downtown core of the city and plans to live there in the summer (rent free mind you). The other day she informs me that she's planning on subletting her room at our apartment from May- July. I'm not okay with this. We live in a fairly low income area in a very large city. I don't feel comfortable living with a complete stranger for 3 months when the whole reason I signed the year long lease at this apartment was because I was under the impression we were going to be living together for the entire duration of our graduate program.

So I'm not sure what my options are. I'm not living with a random stranger for 3 months in the size of city I'm living in. Do I have any kind of power to say no? I don't even have a lock on my bedroom door so worst case is it petty of me to tell her she has to get a lock for my door?
<bot>: Roommate is trying to sublet her room to a random person but I'm not okay with this. What are my options?