Caffeine is a strong substance, and without enough time to let it metabolize, the body goes into overdrive mode, attempting to ward it off, or letting it affect you in ways other than just being ultra alert. On the flip side of the spectrum, those who are used to having caffeine every day can actually experience withdrawal symptoms, most notably, headaches. Once the body becomes accustomed to it, it learns to function alongside it.
How Much is Too Much
There are several factors that play into how much becomes "too much" caffeine for each individual. Most notably, body size and genetics. Smaller adults (and especially kids) will be more greatly affected by small doses of caffeine. While caffeine allergies or reactions can also be passed down from parents. If you're not sure how caffeine affects you, talk to your doctor, parents, or simply keep track of how many beverages you've had, and take note as to how the body responds.
Even those with the highest of tolerances can feel serious side effects when ingesting large amounts of caffeine.
Talk to your doctor about caffeine intakes, or consider cutting back if you're experiencing ongoing side effects.