Ok, we have a few things going on here.
Firstly, any attack needs
LoS. Any part of the attacker's base can see the target's base. You have differences of base size and in the case of intervening models the attackers base size doesn't matter. Only the target and the model inbetween matter.
Terrain also gets in the way and the important part is the hill. In any game it is best to chat to your opponent about what counts as being on the hill and other terrain effects. You said "had moved her up onto the back side of a hill". Were you on the hill or not? If you were on the hill then the rules are
rules wrote:When drawing line of sight to an elevated target, ignore intervening models on terrain at least 1" lower than the target
So if you were on the hill you were effectively in plain sight but at least would have had a +2
DEF bonus from the elevation. The thing is hills do not block line of sight unless they are really tall, check the terrain rules to make sure you understand this bit as it is a common mistake.
Finally you were shot by the Revenant Cannon Crew. They have a special rule, Ghost Shot. "This model ignores
LOS when making attacks with this weapon." So at that point, as long as you were within 14" of the cannon you were in danger.