I dont want my 20th birthday to come

I am turning 20 the next month. It is the first time that I do not want my birthday to come. For some reason I do not know I do not wanna celebrate it. I will just go to work and after that I will buy food from my favorite kebab shop. Three months before I was hoping that I would celebrate this birthday with many friends in a nightclub. But now I want to spend my birthday alone. One year ago I had jo friends and I depressed, fat, no social life, no interest in my life. But this year I have managed to lose some of my weight, hit the gym 3 times a week and build friendships with new people. What is wrong with me and what advice do you suggest?