Endometriosis?

I've been having sex lately and it can be painful after days after even.
But last night was the worst the pain it was so bad. We were sitting and I was on top. It was like I was having a contraction I can't even describe it. Almost like my uterus was going to fall out of me. Everything burned. I was in the shower and my partner had to hold me up.
I was just in shock how much pain I was in. I looked down and my stomach was huge!
10 minutes later nothing. Im fine.
Today I have a period cramp but its a deep sort of pain.
I've now skipped my period this month. It ended yesterday. (Not pregnant, took 3 tests, two looked positive we were excited took another it was negative, we were very disappointed).
I've had 2 proper periods on time this year. I didn't have a period all year to May then June now nothing.
Has any one had anything like this. I saw my dr and she said I have endometriosis she didn't even say might she just said i do and to come back if sex hurts again. I did and then she said she's referring me to a gynecologist. I hvant told her about last night yet.
I have a child already. How can I have endometriosis? I want another baby soon will this be possible. My partner and I have been discussing it lately.
Im a little scared, worried a whole lot of emotions. I don't really understand endometriosis except it grows on the out side of your uterus not the inside, and I thought you got really heavy periods not light or skipped?
Has anyone had any symptoms like this or know how to have sex with out it hurting?