I'm sorry to hear that Maggie has developed a limp on her left front leg.
Does her limp seem to worsen with exercise?
I understand that she doesn't seem painful with touching her leg but is there any pain when you try to flex or extend any of the joints in that leg?
Did this happen suddenly? If so what happened right before he began limping?
Has she had a veterinary examination and radiographs of the affected leg?
Is she bigger then normal for her breed or overweight?
What are you feeding her?
In big, muscular, fast growing breeds we need to be careful that we don't overfeed and push growth rate. Ideally she should be eating a large breed puppy diet and she should be kept on the thin side to decrease stress on growing joints.
Since she doesn't seem overly painful and given your history it is likely that this is a soft tissue (ligament or tendon) or cartilage problem with one of her joints rather than a fracture.
Common problems that lead to a limp in the front legs in larger breed puppies are elbow dysplasia, eosinophillic panosteitis, hypertrophic osteodytrophy, or shoulder OCD (osteochondrosis dessicans). These all worsen with exercise or excess use. They are diagnosed by radiographs most of the time but sometimes require an MRI to diagnose.
In her case because she feels well otherwise I would be most concerned about a shoulder OCD or elbow dysplasia.
Because her limp is persisting I highly recommend that she see her veterinarian after exercise or play when her limp will be more apparent. They will examine her to look for the painful area and will likely want to take radiographs under sedation. Treatment will depend upon diagnosis but can include rest and anti-inflammatories, physical therapy, and omega 3 fatty acids and glucosamine chondroitin supplements to possible surgery to remove cartilage flaps depending upon the problem.
In the meantime I do recommend strict rest. Soft tissue injuries can take months of rest to heal and won't heal if she continues to return to full activity too soon.
I also recommend glucosamine/chondroitin products (like cosequin-ds) plus omega-3 fatty acids. Adding an omega 3 supplement which is a natural anti-inflammatory will help as together they work synergistically, better than either one alone. These supplements can take 6 to 8 weeks to see an improvement with their use. I recommend an omega 3 fatty acid dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example an 80 pound dog could take 1600mg of EPA per day.
Best of luck with your girl, please let me know if you have any further questions.