My ex-flatmate won't give me back my things!!!

Hello,
I have a problem with my ex-flatmate who I shared an apartment with in Madrid for 6 months.
When I left, I gave notice and I had paid the money for my final month, but left a week before. The new tenant moved in as soon as I moved out, duplicating payment for my room for 6 days. My flatmate was not the landlady, but was the only one who dealt with him about the issues in the apartment (the contract is in her name). I sent her a message the day before I left saying that I thought the landlord should return 6 days money to me, which was 100 euros.
I am sure she didn't give the landlord that extra money but instead kept it for herself, she didn't say goodbye to me and got really angry about the message.
Anyway, now that is not the main issue at all, and I don't really care about getting that money back. She had said to me before this happened that it would be OK for me to leave some items there for the summer (I have got text messages to support this). Now I am trying to arrange a time to go and collect my things, and she is not responding to my messages or answering my calls. This is very serious for me as the things I have there include all the important documents I have accumulated throughout my life, including my birth certificate, bank contracts, and all my education certificates, including from my degree. Also there are lots of clothes, photos and books.
What can I do to force her to give me back my things? I can't understand why she is being so unreasonable.
Please help!!
Juliet