While there is no medical proof that stretching works, it is generally recommended that you stretch the muscles and tendons prior to and after activity. Basically, if it works for you, then it works. As for me, if I don't stretch then I will get an injury more often than not, so I highly recommend stretching before and after, and sometimes during, any physical activity.

If you receive an injury to a muscle (strain) then the muscle will tighten up to protect itself from being further injured (this is called muscle guarding and is basically what a cramp is). Problem with this is that it also reduces the circulation of blood to the injured tissues (kind of like stepping on a garden hose) and this causes more damage to the tissues (this is known as the pain-spasm-pain cycle), so in these circumstances light pain-free stretching has been shown to be of benefit. Icing also causes the muscle to relax and help stop the pain-spasm-pain cycle (when Icing, do 20 minutes on and at least 40 minutes off).

For stretching to help with an injured muscle you will get muscle re-education after 1 minute of stretching. Basically, you over stretched the muscle, which caused the injury (strain), so when you stretch it the muscle wants to tighten up to protect itself because the activity that originally hurt it (stretching) is occurring again, so you need to do a light stretch to "Teach" the muscle that the stretching is OK. I recommend stretching for 30 seconds, relax, then stretch for 45 seconds, relax, then stretch for a minute. This teaches the muscle that the stretch is OK and will not injure it more, so it will relax and help increase circulation. I would do this at the end of your activity, then ice it for 20 minutes. That should help the muscle get back to normal.

Basically, if it keeps hurting after you've done all this, then you need to give the muscle and tendon a break, so kick back and relax and try again the next day.