My X-ray showed a 3mm stone in the right kidney however it was not seen in the following x-ray. Why did this happen?
First consideration is how far along you are in your pregnancy when the exposure occurred. If you are early in your pregnancy, there is a higher risk for loss of the pregnancy or genetic damage. The risks are quite low and are also dependent on the next factor.
The second consideration is how much X-ray exposure you may have potentially received. The answer here depends on where you were standing at the moment your mom was being X-rayed. Often, when there is a family member or friend in the room for support, the X-ray tech will have them stand beside them and behind a lead shielded wall/window. If this was what happened in your case, you would have received NO radiation exposure and are completely fine in regards to any risk from the X-ray. It would be unusual for you to be allowed to stand in front of this protected area. If you were, the X-ray tech most likely would have or should have given you a lead apron to cover your torso. If you were in the back of the room but outside the shielded area with no protection, you may have been exposed to a minimal amount of scatter
radiation. Outside of having the X-ray performed on yourself, your greatest risk for exposure would have been if you were standing right next to your mother without a lead apron to shield you. This would be highly unusual practice, but even if this was the case, since you were not in direct line of the X-ray beam you would only have been slightly exposed to some scatter radiation and probably not much more than the amount of background radiation your body is exposed to during the course of a typical day.
Your greatest exposure risk would obviously have been if you were having the X-ray done on yourself. This still occurs at times as not everyone knows the moment they get pregnant or may not even realize they are pregnant even as much as 2-3 months in. Direct X-ray exposures in these cases, especially if they have repeated X-rays, are the higher risk cases but the risks still remain quite low overall.
Based on your scenario, I would expect you received a minimal dosage, if any, and your risks are extremely low. If you have further concerns that is something you can better discuss with your gynecologist/doctor about whether you would require additional ultrasound or whether prenatal genetic testing may be helpful.
As for whether CT would give you any indication of whether it's malignant, CT in and of itself may not give you that information unless you had a PET CT, which is a type of CT done with radioactive "dye". Although histopatholgoy results from FNA can at times be equivocal, it probably represents the best test so far to identify what the mass may be and could clear up any question of whether it was parotid or lymph node in origin.