I have a 6 yr old lab. I have always fed them a really good food with the added glucosamine. I just recently changed foods to one without glucosamine. My lab started limping really bad. I went to human vitamin store and got her some glucosamine supplements. I got the liquid flavored glucosamine and the glucosamine soft flavored chew. I know some cannot swallow pills. My dog eats the chews if I slightly warm them. She will not touch the liquid. I would rather put a pill in some meat or something. It is much easier for her to take. I put her back on a food with glucosamine and she isnt having any problems. I learned my lesson about changing foods. My vet said it was ok to get human vitamins for them. He also said it was ok to feed food with glucosamine plus the vitamins.
You need to have your vet examine her. It sounds like arthritis, and in such a young dog, that means there is likely an underlying problem that got it started so soon. Labs are prone to hip displaysia, so that may be the whole issue. If it's arthritis, there are many different anti-inflammatories you can use to help when he has a pain flareup. You should also ask your vet about proper dosages of over-the-counter glucosamine, chondroiting and/or MSM. You can find these supplements already mixed together at your pet store, but it tends to be a lot cheaper to buy them at a human pharmacy. These supplements are a very effective preventive at keeping your dog's joints in good shape for a longer period of time than if you didn't use them, and will hopefully keep a possible hip surgery from being necessary.