E.M.
I would say the reason this is happening all of a sudden is that separation anxiety has begun. It starts around 9 months and peaks between 11 - 13 months, so you're definitely in the time frame for that. When he wakes up and doesn't know where you are, it is upsetting at this stage of development. You're going to have to be patient and help him through this time. I believe that crying-it-out methods when they are going through the separation anxiety stage could be damaging. Go to him before he gets upset, and you may be able to quickly rock him back to sleep. It will get better, I promise!