I had my period about a week ago, and when it was over I started to have lower back pain and lower right side pains and tired really easily. Any ideas what it could be?

This isn't 'ask a ____ free online.' Anyone here can respond.
All we know about you is that you are female.
You could have nothing, or something related or unrelated to menstruation and hormones, or something minor, or something major.
Even being examined in person by a doctor might mean a certain amount of 'call in a week if it persists' simply because symptoms are vague.
A few possibilities could be fluctuating hormones, ovarian cyst, appendicitis, pregnancy, STDs, even just gas and constipation. So make an appointment and cancel it if you get better.

How could it be pregnancy if I had a period?