The same thing happened to me. mine was caused by lack of sleep, excess caffiene, and stress.
Uterine polyps can and do cause spotting between periods. And the ultrasound should help identify them on the screen if you have any.
Yes I've had uterine polyps and I've had them removed many times. I've lost track how many. Most recently last August. It's a simple procedure and the worst of it is getting the IV and getting over the anesthesia for me.
Yes there's pain associated with polyps because the uterus senses it's a foreign something that needs to be expelled and therefore contractions. That's how my gyne explained it to me. I always know when I have them again due to the spotting and overly heavy periods I always get with them.
The only way to know if you have them is via a transvaginal ultrasound followed by a saline ultrasound (where they fill your uterus with saline and do the ultrasound to see all the details better). It's not a big deal, I've had it done so many times that I don't even blink anymore. But I do roll my eyes as in, Not this again!?!