It is different with every person. You might want to consult with one of your Drs about the vits and sups you are taking. You could be taking something you don't need or that is in conflict with one of the others and is actually hurting you. Sometimes less is more. Its very easy to over do Vit B for instance
But back to your original question: A lot of it depends on your history (were you susceptible to colds before?), your environment (is your house over heated still?), add any new house plants? Plus, we are also getting older. It could just be part of that.