Can someone explain my toddler's personality change?

[Maccapaccawacca]

DS is 16 months, has some hearing problems, glue ear etc. Every week we go to a singing group (thought it would help to have loud singing to help his language etc). We have been 7 times. Each week he spends the entire session stood on my lap, facing me (away from rest of group), clinging on, burying face in my neck. If I attempt to join in with any of the actions he clings harder.
He doesn't seem unhappy during this time (otherwise I wouldn't keep taking him) but very shy and reserved. That is pretty standard for him though, quite a reserved boy - I put a lot of it down to his ears.
My childminder also goes to this singing group (DS is with her 2 short afternoons a week)..so she has seen his typical behaviour.
Anyway, today she took him to the singing group instead of me. She said she was so astounded by what he was like she took a bit of video of him to prove it. There he was, running around with all the other boys, 'dancing' and pretending to sing along.
Obviously, i'm pleased he had a good time but why is he so different with me?Surely he should be at his most confident when his mum is around??I'm not a very shy person so i'm sure he's not picking up vibes from me.
I know they always eat better/behave better for childminders/ at nursery etc but this takes the piss mickey?!

Maybe he's just got used to it and he'll be the same the next time you take him?

Thank you for the kind suggestion but

My 2.4 DS was suddenly very withdrawn at toddler group this week when he realised there was a lady (not seen her before) doing some rhymes/dancing etc with the children. Toddlers is usually very unstructured and he is confident and happy throughout. I put it down to this being new and unusual (although I had talked to him about it before the session). He refused to join in even with me alongside despite her attempts to engage the kids with Fireman Sam actions and music (just DS's cup of tea usually). The change came when she got some bean bags out of her bag and let the kids throw/slide them. He was facinated and went immeadiatly to sit by the activity lady and got completel involved.
I think your persistance paid off and the continuity of going with the CM also your son suddenly took the plunge and loved it.

It could also be that given a choice between Mum and joining in, he chooses Mum, but if Mummy cuddles aren't on offer than suddenly joining in doesn't seem so bad. Or that he's more likely to show his real feelings with you around, and with the CM he was essentially doing a bit of fake-it-till-you-make-it - and then really did make it. I know my D can be her happiest, bubbliest self when other people are around, but then loses it when they leave and she feels free to acknowledge that it was actually a bit stressful.

I have a ds and dd with glue ear and one of the difficult things about it is the variation of hearing levels. So on good days they may be up for activities and on bad they may not, and I have no way of knowing which day we're on. And sometimes having a good hearing day means not wanting to do loud things as it sounds odd... It's tricky - could it be that?

At 16 months my DS was Velcroed to my leg. The separation anxiety started fading after 18 months. Still "shy" and clingy when he meets new people at 21 months but after about 15 mins he's chilled out. You could take it as a compliment...