I just had my period end a week ago and now another one just started, what's wrong?

Maybe nothing: Since you are 18, it is common for periods to be very irregular even for several years. However, the best answer depends on more history-when you started, are you usually regular, medical problems, meds, etc. The most concerning cause is the possibility of pregnancy-it can cause more as well as less bleeding.Hormone imbalances, and many other causes exist. If worried, or have more issues, be seen.

Spotting is common: Most women experience some spotting during their cycles at some point. If your are taking the pills, this makes it more common, especially during the first few months. If you are sexually active, check for pregnancy. Infections, thyroid disease, and many medications can cause this as well. So, if not pregnant and if it recurs/persists, consult doc. Good luck.