Extreme calve pain

Question
I run about four days a week, 2-4 miles. The reason I run is to help my basketball and tennis performance, and also because I love the feeling it brings..that "runner's euphoria".
Unfortunately, that euphoria has recently been replaced by moderate to severe pain in my left calve both during and after my run. It's only on the left side, and it's extreme tightness and occasional pain that is sudden and more sharp. Sometimes I can go an entire mile or two before it starts, and other times it hurts from the beginning. What could this be caused by?
Yesterday I ran almost five miles, despite how much it hurt. Now today I can walk with minimal pain, but going up stairs hurts like crazy. Is there anything I can do to help the pain, or possible prevent it altogether?
I always stretch after working out, and recently have added stretching before I run. I heard that it's not good to stretch before the muscles are warmed up, so I've been doing some dynamic stretching instead. It doesn't seem to help.
Also, I have very good shoes that I replace every few months.

I just don't want to have to give up running, but I'm starting to wonder if maybe that's the only answer.

Thank you in advance for your help!

Answer
Chisomo, calf pain is not fun!  You won't have to stop running though!  I definitely know what you mean about "runner's euphoria".  Do the following:

1) Massage your calf.  You, a friend, a family member, or a licensed massage therapist can do this.

2) Stretch your calf.  If you stretch after you massage it'll be warm.  You need to stretch more often than you'll be massaging though so just rub it a little and stretch it lightly at first before you stretch it more rigorously.  Do two types of stretching.  First, do a regular calf stretch 2 times for 45 second each. Second, pull your foot up (dorsiflex) with just the muscles of your leg (no external help) 3 times for 20 seconds.  

Do the massage as often as possible (at least twice a week, but every day is ideal) and the stretches twice a day with one of the stretches before you run.  

The above items should take care of your problem within a couple of weeks. If it doesn't then you'll need to see a physical therapist because it means that you have asymmetry between the right and left halves of your problem and foot and hip problems need to be addressed.  If you have any questions let me know.  I can recommend the best way to find a physical therapist if necessary and can help you if you're not sure how to do the massage and stretching.  

Good luck Chisomo!