I used to get cramps painful enough to cause me to faint before I went on birth control, which made them disappear. Hopefully that will help in your case. The pain came on so quickly and was so bad I wouldn't be able to swallow an over the counter pain killer even if I wanted to.

My OBGYN ordered an ultrasound for me to investigate whether my irregular periods were something to worry about and it found nothing. That was just an external ultrasound but it's also possible to have an internal one done with a wand. I think if you want to make sure there's nothing there you should be able to ask for an ultrasound, especially since they seem to be appropriate tests for something as simple as a painless but irregular period.

So, in my case there was nothing wrong, if that makes you feel better. However, my cramp "attacks" only lasted about 45 minutes and you're saying yours are way longer than that. I'm not a doctor but I feel like an ultrasound would at least get you a more solid picture than a physical pelvic exam.