IUD, can be a reason for irregular menstrual periods, like spotting or bleeding between periods, but that usually happens at the beginning of its use.
There can be noncancerous growths within womb walls, known as uterine fibroids, causing bleeding between periods, pelvic pain during sexual intercourse, low back pain that does not go away, and urinary problems.
Cervical polyps, small, soft, growths that protrude from the mouth of the cervix, can be another reason for irregular bleeding, too.
Cancer of genital organs (cervix, womb, tubes) must be taken in consideration.
Usually, when menstrual irregularity happens for shorter period like in your case there is no serious reason.
This could be a hormonal imbalance due to stress or thyroid gland disorder.