What Causes Pain Behind My Eyes And Mild Headache?

I fell and hit the back of the left side of my head on the stone stairs at my apartment 3 days ago. I was sleepy immediately after, however I fought going to sleep. The next day there was pain behind my left eye and minor headaches. Yesterday everything was fine and I thought I was clear of any worry. Now this morning there was finally pain on the part of my head that I hit and my right eye gets shooting pain. I've been riding the mile to and from work since Monday on my bike and have been drinking a ton of water. However, last night I had two beers and thought maybe the pain is caused by my dehydration. I'm just trying to avoid going to the doctor if I don't have to because I can't afford it. Thank you very much.