Hi Steve !!
Now that you have told me that your own posture might not be correct , It is possibility that you have got the pain due to improper posture ..
Or it might be merely due to muscle spasm ...
The stretches that you can do are :-
1) lying on the matress ,taking both the knees to chest ,holding for 5 counts ,5 sets .go through this site for demo:-http://www.exrx.net/Stretches/GluteusMaximus/Lying.html
2) These 5 stretches would really help :-
Five Fantastic Stretching Exercises
Try out these stretches 3 times a day and your pain will defenately reduce with in 2 days if it is due to muscle spasm ..
Regards,
Radhik