Endorphin high then low??

getrealgirl

I have been pushing myself to reach some exercises goals that I have never done before. I have had endorphin highs in the past, but today after flying around on cloud 9 for several hours after completing my Couch 2 5K run for the day, I totally crashed and just felt for lack of a better word~~sad. Is this normal? I ate well both before my run and after and don't think it was related to diet. Definitely not dehydrated, I could fill a bathtub with the amount of water I took in today. Anyone else ever have this strange feeling?