Hi. I know what you are talking about. I get this around day 8-12. I am on the pill too for endometriosis. I take the pill continuously for several months and this only ever happens during the first month after allowing a period.
Sometimes it is difficult to lie down or sit, because it hurts so much to bend at all. Any sudden movement can cause massive pains. I found drinking a LOT of water may have helped... but maybe just a coincidence.
I had my period 2 weeks ago and have had slight pain the past 2 days... but will probably be worse the next few... Also, I get sharp pains..ummm.... somewhere... to do with bowel movements.. Its not nice.

For me, it feels like my pelvic area is full of gas and causes pressure on ovaries, etc. I have cramps too, but haven't noticed any change of discharge.
I woudl mention to the doctor.. I have had an ultrasound and didn't have any cysts causing it.