Depression after a work out...? More common than we think.

So I've done some cursory research and read some blogs and skimmed. I got curious because exercising has alleviated my stress in the past and made me feel good. I would get a sense of accomplishment from having worked my body and got my sweat on! We all know that confident, boosted feeling. Endorphins give us that feeling, but most of us know that. More recently, I feel frustrated, angry, irritable, mopey, moody and simply "down in the dumps" following a work out. I usually work out for an hour on a bike and try to get 15 miles oh high intensity. If I don't get my goal, I don't care. The success of the work out is that I did what was possible of me. Some days, I might be psychically run down and only able to do 10 or 12 miles, what ever. I also do 30 minute interval cardio videos on Xbox Fitness. I consider and feel that these activities are rewarding on their own, I do enjoy the process of the work outs. For the last few months, after I exercise, I just don't have that awesome sense of well being after completing work out. I asked my primary care physician about feeling sad after working out. Her answer was, "Just keep working out and I promise it will help with your mood." I've lost about 40 lbs since January. I work out regularly and I stay pretty active any ways in terms of daily activity (laundry, groceries, cooking, cleaning, scrubbing, vacuuming, it all really adds up!). My mood fluctuates just as much as any "average person." So, here is the mystery: Why do I have an acutely lower mood for up to a few hours after I work out? Most of the time, I do have a pre- workout shake. After the work out, I am sure to get my after work out shake. They both have vitamin powder, carbohydrates and protein of varying levels depending on need. I hydrate about 24-48 oz during the work outs. I hydrate before and after. I noticed many people blaming "refined carbohydrates." I am for the most part a naturalist when it comes to most of my diet. I usually have ONE soda that I sip through out the day, over several hours. So if none of my patterns have changed in the last few months, why would my mood suddenly become worse after exercising? AND people also seemed to blame stress and subconscious stuff, hypothesizing that exercising was like popping the lid off of a can of worms. The thinking on that seemed to be that working out gives the brain a break to sort through stuff, and then when one is done with their work out, they are in a vulnerable state and susceptible to distress from emotions. I have yet to find an answer that is satisfactory and based on science and not pop culture.