Normally, our lymph nodes are dealing with what are called 'toxins' above all the time. We release them throughout the day and the lymphatic system processes this 'crap' for us. However, during a sports massage, the various releases of both lactic acid and 'toxins' in knotted muscles can sometimes lead to the lymph nodes having a greater workload for a few hours.
Hence both the groggy feeling and the need to boost the system by providing enough water to help dilute everything. It's fairly normal to feel a little tired or groggy after a sports massage and it does improve you in many ways - flexibility, reduced tightness, increases in muscle function and of course relaxation. The more regular the massage, the less likely the effect on your system as there is less work for the poor old lymph nodes to wade through.
A good therapist can also find areas of tightness or weakness of which you may not be aware, so all in all, sports massage should probably be a part of all our training programmes.