as people have already suggested it sounds like it is down to the muscles on your back not being used to it. I'd suggest that you pop down to your local gym and see if they can help with some back exercises that will help strengthen your lower back muscles (or maybe someone on the forum here can do it as well - im not particularly knowledgeable of specific exercises!) Dont forget though, that if you strain it too much during the day, you oughta spend longer resting it at night.
How did you lose so much weight, its very impressive so well done you!