Because so much of being depressed is hormonal, I think it is totally possible that you can go from being "perfectly happy" to "utterly depressed" overnight.
However, usually there are situational causes too. I think perhaps you have been through so much that you have been in 'survival mode' over the last year or so. Now that things seem to be calming down perhaps you are finally starting to subconsciously deal with all the heartbreak of your recent life. This can cause depression, anxiety, PTSD, among other things. The important thing to realize is that it is normal and perhaps even a healthy reaction to life's tragedies. Sometimes we need to take time out to grieve, even if we think we are okay. Make sure you have someone to talk to, professional help if you feel that will help, and know that you are not alone. Your sadness is because you are a caring and empathetic person and it is understandable that you might need some time right now to deal with everything that has happened to you.
I wish you the best of luck and all the support you need, my dear. Feel free to PM me if you would like to talk - I have been dealing with depression and anxiety for years and i know how much a sympathetic shoulder can help.