I think its quite normal. When dc1 did it, I freaked slightly, as he went from eating absolutely bloody everything (like yours, OP), to almost nothing (a pretty similar list) in the space of about a month.
Initially, it was a battle of wills, which wasn't a) healthy and b) fun.
Then I read somewhere (probably on MN) that it can be an evolutionary response, as it is around the 18 mo mark that babies would have been/could be old enough to start foraging for their own food....so they sort of 'shut down' to a very safe and limited selection.
It's probably bollocks, but I liked the theory :)
DH and I constructed a controlled response that went like this:
Breakfast - always easy, ds would always load up on breakfast anyway. So we actively controlled how much he ate so that he would definitely be hungry at lunch.
Mid morning snack - cut it down to half size : half a banana, or a small biscuit, and water not milk.
Lunch - 1 item ds would eat (in smallish quantity) and rest items he might/might not eat in various shades of challenging. Dessert would be fruit/yoghurt but only if he had tried at least one of the non-standard items.
Tea - small (like mid morning snack) or half cup milk.
Dinner - 1 item ds would eat, rest stuff he might/might not. Dessert fruit/yoghurt but he had to have tried at least one thing apart from his preferred item.
Bedtime milk as normal.
Once we got this going, it only took about 2 weeks to get him back on track, albeit with a slightly less adventurous palate than before....and now at 7 he is turning into a real 'foodie', and recently opted for a very surprising salmon en croute/spelt salad/aubergine salad/quiche combo at a friends house, over the kids pasta, sausages and cheese menu she had available, so I'm glad we stuck at it :) :)
Good luck!