You remain still: no matter what, you will prove to her that you're not a pet she can boss around: you're a person, with your own will and desires. If to prove that, you need to disobey what's likely a clear command from a colossus, so be it: you're brave enough to deal with any consequences.

Yet to Shina, that's not what this looks like to her, you are like a stubborn little prick, who for some reason refuses to drink even though you're not well. She sighs, and in a hesitant tone, she speaks:

"I...don't why...you keep refusing, even though you need this!... I'm sorry, but right now, I can't accept a no for an answer here."

She then pushes you against her, the top of one of her fingers pinning your head against the orifice, where the pouring milk quickly covers you, threatening to drown you unless you suck it up.

You understandably try to escape, yet her massive finger doesn't bunch an inch with your efforts. You manage to hear Shina's voice, who in a worried tone, exclaiming:
"I'm sorry, I don't know why you don't want to drink my milk, but this is for your good! I'm a responsible owner, I care for you! And if that responsibility means that I must force a disobedient tiny to do something he doesn't like yet it's necessary for your well-being, so be it! Just drink up, please! I'm not enjoying looking at you squirming like that, nor forcing you to do this, so again, I implore you to drink, so we both can finish this unpleasant situation for us both as soon as possible!

Realizing that the girl will only stop having you pinned like this against her nipple, and the honest threat of drowning filling your mind, you have no other choice but to obey, to your greater dismay.

You began to suck, and even though it was rather tasty and energetic, you still don't enjoy it: she got you to obey her again, something you don't appreciate in the slightest. Besides, it seems your efforts were misunderstood; you will have to find other means to prove to her your true status.

As for the giantess, you can hear her sighing, happy that the situation resolved itself. In a weary tone, she speaks to you, saying:
"Finally...was it so difficult, my tiny pet? Was my milk so bad as to be undrinkable? By how much you keep drinking, I think not...please, I don't want to do anything like this again; breastfeeding a tiny one is supposed to be fun times for both, not an ordeal. I wish that from now on, when I offer you something, you take it instead of stubbornly resisting, it's not like you can do anything to defy me if I so want it...doesn't make it enjoyable though..."

She has a point there, and maybe, just maybe...that might be enough for you to completely reconsider how you're going about the situation...

Yet that could also mean surrendering to your current status as her pet, something you're adamant about for the moment to accept. 

But those are thoughts to delve into another time, for when you're full, Shina immediately shoves you back inside, with her moans being weaker than ever before: it can summarize that forcing you to feed left her in a rather sand and sour mood.

As you once again find yourself in the now empty gland, a potential opportunity for an escape attempt lies ahead. Yet she's fully awake and focused on you know, so it might not be worth it. 

Lastly, you might feel bad at the effects your little "tantrum" as it can most easily be described as had on the girl, and so you might prefer to make it up to her, cheering her up if just a bit. 

In the end, what next step you take is up to you.