I know caffeine withdrawal can cause headaches & nausea, but can someone who is used to having caffeine in small amounts & who normally drinks just water have those issues if they drink a lot more caffeine in a week than they are used too?

For most people up to 4 cups of brewed coffee , containing about 400mg caffeine is safe ,though sensitive people could react to lesser amounts .Side effects include jitteriness, headache, insomnia , irritability, fast heartbeat and tremors .Shortening the brewing time , reducing intake of coffee as well as tea and sodas ,gradually , would help to avoid much of this .