renesmee74,
Fortunately, injury to the cervical arteries is quite rare and most of the cases are from significant trauma, such as a traffic accident. But, there are known cases that do occur from manipulation of the cervical vertebrae. It is still not known if the manipulation actually caused the problem, or if the patient actually had a spontaneous onset, which just happened to coincide with an episode of manipulation. And, another factor is if the patient has other underlying factors such as high blood pressure, arteriosclerosis, and atherosclerosis, and how much these play into the overall problem.
Unfortunately, sometimes the injury to the intimal lining of the artery may not be significant enough to cause immediate symptoms. The injury can cause problems inside the artery, which then take time to build up a clot. So, in mild injuries, the patient may not have any symptoms immediately.
So, if you are concerned, you may want to call your mother's physician for advice on how to proceed. Again, injury to the cervical arteries is rare, but not unheard of. This is something that you will have to go with your gut feelings about.
Hope your mother is okay and doing well. Good luck.