It is rare for them to go bad, unless they are left in a very moist warm place. Keeping them airtight in the fridge is best. What more likely is that the tea wasn't throrough mixed and you got a part with too much of one paricular herb than it is intended. For example, too much confrey can make the taste too strong. So mix the herbs with your hands until all parts seem equally concentrated. THen try it again, and it should be fine.