The OP is happy with his decision and that is all that matters. :shrug:
But, I don't think having other Photographers being of the opinion that the OP has in their opinion given his work away too cheaply is unexpected. Most Photographers take pictures because they enjoy the process, and having someone appreciate the resulting pictures is definitely an ego boost. For some, that is enough. If someone is asking to use your image commercially, then again that is an ego boost, but it is also an opportunity to make some money from images, if for nothing else, to help pay for all that expensive gear. :shrug: Each situation is different, but it is usually the Photographer being asked to give their services (images) for free. The person usually asking is being paid, and the people producing whatever the image will be used in, magazine, poster, website etc, is also being paid. Without any images though, whatever it is doesn't get produced. So the image has value, if for no one else but for the people in the chain to produce the product.
Reading this situation, I initially thought that small wooden Calendars would be expensive, and so the OP may not have been wise to give their image for nothing, but when it is an exhibition that is going to run for many weeks, and people will be charged 'Standard admission prices for adults £8, children aged 5-15 £3.40, under 5s free and concessions £5.60.' Then it looks like everyone will be paid except the OP.

And anyone else who gave their image away for free.
I read a magazine article last week where a Pro Photographer was being asked to give his pictures to a company that he had been doing 'paid' work for for many years, for free, he declined their kind offer. But people, and especially business, are trying to get things for free to save money, if they can get images for free from other sources that may, or may not be as good, but cost nothing, they are going to use them, or at least use that as a reason to pay less for images. It is understandable that people who may be Pro Photographers, or aim to sell any images, will at least have an opinion about someone giving images away for free that they perceive to have value. If nothing else to stop the person posting to not be taken advantage of. :shrug:
Like I said though, if the OP is happy with that then take the not unexpected comments on the chin and enjoy the image when it is unveiled.
You did at least negotiate a free entry so that you can go and see your image didn't you?
