Why am I suddenly feeling Depressed and Lonely 4 Months after the End of my Marriage?

My marriage finalized Mid Feburary of this year. My wife and I were married for 4 years. Surprisingly for the first 4 months, Up until last week, I have not been depressed. It's actually been pretty exciting being single again. I felt young and energized. Hanging out with my friends again felt great. I felt Free . My birthday was 3 days ago, and on my Birthday, everything was going great.., suddenly mid-day, I started feeling depressed , it just dawned on me that I'm alone. Then I started thinking about how I would never be with my ex wife again. Did it just dawn on me? Over the past 3 days I've felt almost sick. Suddenly , now my home feels so empty. I feel like my soul is gone! I want to drive her house and talk to her again. Whats going on? Today has felt like the most lonely day of my life. Am I suddenly getting depressed? Whats wrong with me? I just feel dirty, lonely, and dark.