Karma is extremely difficult to understand because in every situation there are numerous karmas working at the same time and also numerous circumstances affecting the result.
In some way you can compare the complexity of karma with the complexity of gravity. Every single atom in the universe is subject to the influence of gravity from every other atom in the universe. Of course the closer they are the more influence they will have.
In karma, the strongest karma will have the strongest effect in the outcome, but other karmas will also have an effect. Only an enlightened being can fully understand it.
For example you may have the karma to be born as an animal and also have the karma to have a live free of suffering, then you can be born as a pet of a very rich family that will take the best care of you. Or you can have the karma to be born as a human being and also have the karma to have a live of suffering, and then you can be born in a violent and poor country and experience a lot of suffering.
Karma, understood as “cause”, can only have a limited “effect”. There is no such karma with unlimited effect. Once the effect of a karma has mature, it won´t happen again.
Personally, I like the definition that good karma is everything that helps you get closer to enlighten while bad karma is everything that moves you away from enlighten. Winning the lottery or getting cancer are neither good nor bad karma per se. If winning the lottery distracts you from your spiritual practice, makes you forget about the beings suffering around you and males you pride, then it is bad karma. While if getting cancer makes you think about the suffering of others, opens your heart and makes you a more patient and considered person with the people around you, it is good karma.
You can also say that good karma is what allows you to learn from every situation and makes you grow, regardless of what that situation is. Bad karma is what makes you react is such a way that it will hurt your spiritual development, regardless of what the situation is.
The advice would be not to worry so much about what happens to you, worry about your response to those challenges.
I hope this helps you.