Hi!
The risk is pretty low. The concern is that cats often use sandboxes for litter boxes.
If you are certain that no cats have been visiting, you are fine. If the sandbox did have any cat "prints" you might want to ask your doctor for a toxoplasmosis titre. This is probably overkill, but might be reassuring.
I have never seen a case of toxoplasmosis in 20 years of OB, so its pretty rare. Just a bit more reassurance.
Good luck!
Dr B
Maybe you should contact your doctor, And get a test, Have you noticed any changes in your pregnanacy after the incident?