Nann1 I think, as others have said, GS' behaviour could be a 'blip.' but painful for you. I see my GA once a month on average and he is 21 months old. He does seem to be a little shy by nature and even though he now remembers his ' NanniO' he needs time to warm up to me each time. This always makes me sad inside, but once he has got through the initial shyness, I feel he quickly becomes affectionate and seems very relaxed and somehow familiar with me very quickly. HOWEVER, he does not like being left with me. He was fine when he was younger, but in recent times he has become quite distressed if I have offered to take him out on my own / if mum or dad go out of sight for any length of time.
I tend to think your GS may just find it difficult to be out of the safety of the home routine with you when you visit.
I have a son who is on the autistic spectrum. The early signs - at toddler -hood were lack of eye contact, walking on tip-toe, being very 'regimented' in play, e.g. particular about lining up toys and being very repetitive in his play patterns, and not coping in any busy social situations. If GS' nursery have not picked anything up and parents are not concerned, it may be just a developmental stage / phase.