She is probably not in pain, if you are inserting the needle in her scruff, but she may, indeed, be restless with the process. We give our Sven sub-qs every other day (150 mls), and he is always restless and squirmy! It's never something we look forward to, but we tell ourselves (and him), and we're not hurting him, we're helping him, and once we get thru, we set him down and he's good to go! The only time we stop short of the 150 ml is if he starts yowling, and the last time that happened is because he needed to use the litter box
.
Anyway, warming the fluids is very helpful (in a pan of hot water, not the microwave...I actually heat a bowl of water in the microwave, then set the bag in the bowl of water), and some people actually feed their cats while giving fluids to keep them busy! Since there are two of here to do it, hubby holds the needle in place while I stroke Sven all about his head and under his chin to try to keep him from getting too restless. Some people wrap their kitties up into a kitty burrito with a towel and others use a cat bag so they can't go anywhere, and it probably makes the cats feel safe.
With time, most cats just get used to it and "go with the flow" no pun intended. (my Sven being the exception to the rule!)