I thought exercise was good for mental health?!

I have started running each morning. They say it's supposed to make you feel energised but I am exhausted. I started running three miles from last week, had the weekend off and started again this week and I have no energy at all. Overdoing it? My mind is sluggish which tends to cause me anxiety and in turn OCD. I also find the tiredness and sluggishness quite depressing and it seems to sap my confience. I'm not sleeping as well either! I don't feel properly tired each evening. Maybe the sluggishness stops me tireing my mind out during the day so I'm not tired at night or am I talking rubbish? I am reluctant to stop running so perhaps I could do fewer each week and build up. I feel I should be able to manage it though. I'm not superwoman but it's only three miles!