Hamstrings are chronically weak as we don't use them very much, so they are easy to get tight and strain. You should try and warm the area (I.E. take a hot bath and soak for 20 minutes) and then stretch the hams and and glutes. The stretch should be pain free and static. Hold the stretch for 30 seconds, then relax and shake out your leg, then stretch and hold for 45 seconds. Relax and shake out your leg, then stretch and hold for one minute. It takes a minute of stretching to retrain the muscle to relax. You need to concentrate and make sure you are relaxing when you stretch. Do your exercises and take it easy. If it hurts, modify your activity.
After you are done with exercising, do the stretching routine again and then ice the area for 20-30 minutes. While the tissue is thick, the ice will still cool the area and help to reduce swelling. You also need to start a hamstring strengthening program to keep the problem from returning.
This could be something else though, such as bursitis or other problems. Hard to tell without doing an evaluation on you, but the best thing you can do is a good all around stretching and strengthening program of both legs and your back, and when you are done then definitely ice the area that hurts. 20 minutes is normally the maximum time you would ice an area, but with deep tissue you can push it to 30 minutes on and at least 45 minutes off. The tissue should be warm before you ice again, but if you sit on the ice you will also get some compression.
You can ice all day (I.E. 20 minutes on, 40 minutes off) and it will help reduce the swelling to the area. Keep an eye on the injury and if it doesn't seem to get better then it may be something else. Bursitis takes a LONG time to heal, and if you are sitting on the bursa sack it takes even longer, so make sure you have adequate padding if you need it.
If things don't get better then you need to find a doctor. If you are near a place that has a sports team, see if they have a Athletic Trainer (Sports medicine) and see if the trainer will evaluate you. They may be able to tell you for sure what is wrong and probably won't charge you anything.