That won't always be the case. I can quite understand you being on set units for a while till you and doctors see how you settle down with limited variables in the sense of set insulin and set food amounts. But eventually, once you've learned some basic carb counting and dosing rules novorapid gives you a large amount of flexibility with what and when and how much you eat.
But that's for the future. For this afternoon, I agree with
@urbanracer 's point about the shape or pattern of insulin. It's an important point generally to keep in mind which will stand you in good stead for the rest of your T1 career, realising that insulin isn't "fire and forget", that it'll be doing it's stuff for 4 to 5 hours, peaking after 1 to 2, and realising how matching digestion rates to that pattern can help. That's why guestimating the gi of your meal plays a part. And why you can still have substantial drops in the last hour or two of action.
With experience, eating again during that 4 to 5 hour action period is totally do-able, but the main thing to think about is that if the second meal has carbs which you need to dose for, you'll then have two lots of insulin in you working in tandem but with different peaking times, which can become unpredictable for newbies. It's called insulin stacking. There's nothing intrinsically wrong or bad about it, but, like I say, needs a bit of attention, so you should probably avoid it at this stage until you're more familiar with things.
Your nibbles situation could work out in a number of ways.
If it turns out the 6u for the sandwich was too much and drops you, you'll be needing some more food anyway. If you get anywhere near hypo, use fast sugar, not nibbles, but if it's more of a gentle slide which you just need to tail off a bit and lift before you hit 4, some carb nibbles might be just the trick to bring it up a bit.
If the 6u was correct and you're not sliding, then, generally speaking people like DAFNE will say you can get away with about 10g between meals without bolus, but that's obviously a really small amount, like a digestive biscuit.
So, if the nibbles are going to be more than 10g, you're getting into territory where you're going to have to think about taking another shot, and then you're insulin stacking. As you're on set doses in these early stages, your dsn probably won't be wild about that idea, but, obviously if you've been taught enough already by your dsn about how to deal with such situations, then follow that advice. Personally, if I'm insulin stacking, I'd generally think about how much I'd normally bolus for that meal, but then tail it off a bit to allow for the fact that I've still got some active insulin on board.
If the nibbles are carb free, then there are no real issues. If you're talking about olives, nuts, ham, cheese, salad , you'll not need to bolus for those, so go for it. Sure, there can be issues about needing to bolus for protein in some circumstances, but let's not complicate matters!
Personally, seeing as you're on set rates, I'd be inclined to stick to no carb stuff, unless you've been taught enough to calculate bolus for additional food, and are confident enough to deal with stacking issues.
Like I say, this won't always be the case: bit of experience under you're belt and this'll be second nature, but for this afternoon, be cautious about it.
