According to Buddhism, everything that happens, happens for a reason, which is due to karma. How does Utu Niyama, Bija niyama fit into this?

I've listened to many theravada buddhist sermons and something that is always mentioned is how everything that happens, happens for a reason. Does that mean that if we get sick, it's our karma to get sick, but then we get better because of our good karma as well? How does taking medicine fit into this? Does the temporary relief that medicine gives us our good karma as well? What if one never takes medicine thinking that they will get healed according to karma? I've personally found great comfort in the fact that everything happens according to karma. So I feel really confused about the other 'niyamas' as well, which recently came to my mind. How does Utu niyama, Bija niyama relate to the fact that everything happens for a reason? Edit: accesstoinsight.org/tipitaka/mn/mn.101.than.html The Devadaha Sutta answered my questions. Thank you so much to everyone who responded