Would gallstones definitely cause problems after eating dairy or greasy meals?

yes, possibly: Fatty meals cause contraction of the gallbladder, in order to release the bile stored within, because bile aids in the digestion of fatty foods. If you have gallstones, the contractions can cause a gallstone to move into the neck of the gallbladder and get stuck there or in the common bile duct, which causes pain and will require either surgery or endoscopy to remove the blockage.

Not necessarily: Have you seen a doctor or had an ultrasound to check for gallstones? Just having gallstones does not always mean they are the source of a person's concerns. Yes, eating fatty or fried foods can cause upper abdominal pain ; nausea in some people with gallstones. Needs a doctor's evaluation.

not necessarily: Most gall stones are silent and would not be affected by diet. If the symptoms come on after fatty meals, then you may benefit from surgery(cholecystectomy) to remove the gall bladder.