This is purely Tofu (but I still think the reason is at least partially is that she is just a tsundere. she just acts like she would be very upset. and she eventually leaves if I am really insistent with my torture. I.e. lightly petting her head and back).
By the way, I saw videos (I start to doubt videos about cats… a vet said cats barely drink. mine drink A TON, I refill their bowls so often…) and they said we need to wait for permission to stroke a cat’s belly or else it will claw or bite or whatnot.
Well Tofu is our grumpy cat, she bites me in no time if I touch her when sleeping (lately not always), I need a tad more when she is awake but she is still trigger-happy. But petting her belly when she didn’t initiate that (she NEVER does such a thing. Caroline is that type. Pie doesn’t show her belly, she just soaks up all the attention she can get. I can do whatever with her for almost any long, she is fine with it)? Easy. Careful at the point where she is very trigger happy if I go even near to her but not worse than any other part of her. I successfully petted her belly many times.
The other two never bites and Caroline only would claw if I would held her for too long (I mean when she leaves the ground. she doesn’t like that for more than a few seconds and we have an understanding) but I never tried that. When she starts squirming (after her few seconds), I put her down very soon. Not always immediately, it’s fun and I am not so nice so she won’t be happy for 2 more seconds. But I rarely do that, I prefer Caroline fluffy, soft, compliant (to that tiny extent she is).
So nope, belly petting is pretty safe with our cats at any time. Except Tofu when she is most upset with me and the world (I know it’s not personal as she dislikes the other two as well. she tolerates my SO the best but he doesn’t touch her as much as I like to. cats are for petting! I couldn’t resist. and tsundere Tofu is fun. sometimes a tad bloody, oh well. her claws are super sharp, she doesn’t really want to hurt me. often. sometimes she bites harder, still NOWHERE near her strength but usually her bites are feather light touches 
so she is totally a tsundere and not even the more violent type. but claw control is harder than bite control).
I have no new photos, sorry.
That stupid ear cutting, looks utterly horrible.