Only a vet would be able to check whether something has gone awry. Best thing I can recommend if you can't consult a vet would be to watch for any change in behavior or unusual appearance, particularly near the site of the injection, until you can reach out to a vet.
I don't know where guinea pig subcutaneous injections are given, but generally with a subcutaneous injection (not specific to guinea pigs), if the needle were placed incorrectly and the fluid didn't go into the subcutaneous layer, it would be going into a muscle ... if you're using a syringe, that should feel different as you press the plunger.
Also, while I don't have experience with guinea pig subcutaneous injection, I do have some experience with human subcutaneous injections ... different species and all, but it's the closest I can offer ... When I've had subcutaneous injections, there isn't always a bubble visible ... sometimes there is; sometimes there isn't - in cases where there wasn't a distinct bump from the injection, generally I could feel the fluid moving further to the side(s), where as if it stays more localised, there's more of a visible/tangible bubble ... both subcutaneous, just how much the fluid was able to spread within the layer. Not sure if that's helpful.
Hopefully you can reach out to your vet when they open and they'll be able to give a better assessment and recommend course of action.
You're a great piggy owner for going through this for Sandy. I wish you and her the best with this.