endometriosis?

So, I'm thinking i may have endometriosis. I did google the symptoms and I match up with the majority of them. I thought I had PCOS but when I went and got an ultrasound, they couldn't find any cysts on them... though I literally pop a cyst 4-5 times a week. Something is off with my body and I cannot figure it out. Ovulation all the way through my period and a little after, I have random cramps and pain. I have irregular periods though I've been on BC for over a year and have issues with nausea and constipation for pretty much no reason, as well as all the other normal pains. I know the symptoms, but I don't exactly know what endometriosis IS. Like, how it works. I did google it, but it didn't make much sense for what was really going on. Can anybody with it or who knows a lot about it tell me what it's doing to your body? And what to do about it?