My sister is on her fifth month of pregnancy and she has been experiencing pelvic pain that started on the second trimester. Though she doesn't have any infections, what are the possible causes of this pain?

When I was pregnant, I can remember that I also had the same problem but for me, I felt it on my back. It was so painful and uncomfortable that I ask my husband to pull my legs so as to relieve the pain. I even take in pain meds because of this.