L.A.
She is an aging dog and as they begin to age, they start to lose bladder control. Does she act embarrassed or like she's scared she's going to get in trouble? That could be a sign of bladder control loss.
Talk to the vet, if it's not an allergic reaction the the injection (which I've never heard of that type of reaction and I worked in a vet, but anything is possible I guess), she should get a good geriatric blood work screening to see if anything else is going on.
Perhaps she's sensitive to the grass or fertilizer you may have on it, and prefers not to go outside right now. Try getting a puppy training pad, and showing her where it is, so maybe she will at least limit her accidents on top of the pad.