Laptops in themselves are not overly heavy but it is natural to
include your other bits and bobs in the same bag and then..."I better
take my power supply too!". The ultimate payload on your shoulder can
be enough over time to skew your frame. You can end up with one
shoulder lower and further back than the other one - not surprising
that this can cause pain and be a factor in RSI.
A better solution is to get a Wheeled Laptop Bag. It
doesn't have to be tartan and accompany you to the shops! They
generally look very business-like and once you start using them you
will quickly feel the benefit.