Bad Day

Today was supposed to be a 4 mile tempo but it ended up being.... a disaster. It started off great with a sold 2 miles in 12:00.  Then..BOOM.  An excruciating pain in my right side- i tried to fight it off, but i was forced to walk for 2 min.  I then picked up running again and ran in the rest of the way at a fairly easy pace finishing with a time of 27:30... 3:30 slower than my desired time.  I am now questioning why I even run, when I hated it so much today.. Anyone else have these kind of days?