My back aches when i take 1500mg of glucosamine, but now the pain was gone. what could be happening in my back?

Impossible to say: It is very unlikely that taking the glucosamine caused your back pain. But while it has some support for help in knee arthritis, glucosamine generally doesn't help back pain at all. At your age, a strain from activity likely caused your back pain and, as expected, it has healed itself. Walking and back stretching and strengthening exercises may be helpful to prevent recurrences. Stay well:)