Help, my site is attaked in database

Hi.
2 weeks ago I faced some errors in my website:
Fatal error: Out of memory (allocated 193724416) (tried to allocate 394 bytes) in public_html/wp-includes/pomo/mo.php

It was strange. The memory limit is 300 MB and its enough. I tried every solution to increase the memory limit but the problem still exists.

I decided to change my hosting service and when I opened phpMyAdmin to backup my database, I realized that there is a problem in DB.
The wp_options table was about 140 MB ! It was about 2 months that I started this website and 141 MB is very very much for 2 months.

I started an investigation and I found out there is 22460 records with option_name = /oakley-sunglasses-for-women-c/3SY303/ and option_value contains 5787 characters including HTML and links and spam materials.

My DB was under attack. A bad one!
Please help me to understand how this is happened. Witch passwords is needed to be changed? What was my fault? I’m a developer and I don’t want this happen to the other customers.