I don't have experience giving IV's to horses but I've given over a hundred to dairy cattle.
It could be an injection site reaction, inflamation from the needle.
Did you give her Dextrose (sugar)? If you did and any missed the vein then it can cause a nice little infection from the bacteria consuming the sugar.
It is from dirt or perhaps a little bacteria that entered where you gave the shot. Horses are very suseptible to dirt under the skin.
It will eventually go down, but next time be sure you clean the
spot with alcohol before you give the injection~that keeps
the site cleaner and you will have less incidence of knots.
FH is right it is from something you brought in or a little fluid that got by your needle. I have the same thing on a little filly that ran thru a fence. The vet IV'd fluids to her and some got by. It will go down. Our vet said that we could drain it if we wanted, with a needle.