I ran 7 miles last week, and have not been able to run even a half-mile since. Why?

I am a beginning runner (4 weeks). My longest run up to this point was 6 miles. Before my 7 mile run, I was experiencing pain in my calves. I decided to apply topical magnesium (to relieve the pain), and run regardless. I ran my 7 miles @ 6 mph. I figured it was only a mile more. However, my body wanted to quit at 6.5 miles I ran the next quarter mile @ 8 mph, and then my heart began to beat too hard for comfort. So, I slowed to a walk for an eighth, and then sprinted the last eighth. I was pretty exhausted that night. That was expected. However, since this run, I have been unable to even run a half-mile without intense physical pain (more than I experienced after my 7 miles). Is this normal? Is there a reason? If so, is there a solution?