I had sex august 13 my period is usually the 27 but it came the 29th it usually last 3 days this time it lasted 5 days ...now im havin back pain etc?

Get a pregnancy test: It is likely that this could just a delayed and longer period due to factors such as stress/ exercise etc. There is always a chance that you may have conceived as well - if this is the case then the longer bleeding may mean, that the pregnancy is aborting. There is still a risk of an ectopic pregnancy as well. You need to get a pregnancy test and see a doctor if your pain and/or bleeding increase

GYN exam: is necessary to diagnose your issues. Your cycle may be changing or you may be suffering from GYN illness: you must be examined. You are definitely having menstrual cramping: OK to use Advil or Aleve if not allergic to these medications- call for a GYN exam today!