That happened to me when I was giving Mishi Sub-Q's. As TopCat3 said, don't beat yourself up over this. It happens. Each time giving Sub-Q's is different. I noticed, too, that some days the fluid would flow slowly and other days it seemed to flow in very quickly. Some of the tips here should help.

Just know you are doing the best you can to help your cat get better and each day is different.