What Causes Recurrent Mood Swings?

I m a 37 yo man.For a few months I ve been affected by weird changes of mood. Every 3 to 4 weeks I loose interest in everything, but only for a couple days: 4,5 or 6. Then I am myself again and in about a mont it starts again. Normally I m an outgoing person, love to hang with friends and family or send time with my girlfriend. I ve sharp sense of humor, love to ride a bike etc. And then one day I just wake up with an empty mind. I don t feel like doing anything, I see or talk to/text nobody. I cancell whatever I had planned to do with others. I just sit and do some very minor repairs or sit and watch TV and think of nothing. It doesnt bother me that much but my close ones are very upset. What is it? Why does it happen every month for a few days? I ve never heard of anything like this. It s never happened to me before in my life, although I used to be alone/in and out relationships and maybe I just havent noticed... Is it a psychological/psychiatrical problem?