I wonder if you have a leaking ovarian cyst? They can be very painful.
Have you had a basic ultrasound to check for a cyst on that side?
If not, and the pain gets worse please visit A&E for an intra-muscular painrelief injection (you might need something that strong) and also get a scan to see what's causing it.
If you're pretty sure it's ovary/gynae area related.
Another possibility of sudden and severe pain can be a kidney stone or kidney infection. with an infection it will show up in a urine sample, and you will feel shakey and sickly with it, but with a kidney stone beginning to move, you won't feel unwell as such just in tremendous acute pain. If the pain is more at your waist area if you grip the sides of your waist then roughly where you thumb will be on your back..... that is more likely a kidney stone and you must drink and drink and drink more water than you ever will feel comfortable drinking to try and flush that beast on its way. Don't wait at home, get to hospital pronto. It can quickly get to a 10/10 on the pain scale.
If there is a possibility you might be pregnant, get checked for an ectopic pregnancy..again a scan and urine test.
If it's endo then that is more likely to begin to play up when you are on a period than inbetween periods, so if you have recently started or are just coming to the end of a period, it could be endo related.
If the morphne is hardly touching the pain please get to hospital. Don't wait till Saturday it could be a sign of something that is potentially very serious if left alone. Just because you have endo don't assume it is endo and immediately right off all the other possible reasons for sudden onset severe pain.
It might be something like a leaking cyst in which case they may want to just let it calm down by itself and check on it in a few weeks, but still give you adequate pain relief, or they might decide to help themselves to your pesky cyst.
Kidney stones (most of them) will pass eventually if you give you body enough liquid to flush it out. You won't feel like drinking anything, but it is a case of force feeding youself against your own better judgement. But don't do that till you have been x-rayed to check for a kidney stone.
Other things it could be are a twisted cyst or twisted ovary, or a kink in the intestines, a hernia, or even a torn adhesion, an infection in a fallopian tube, PID and so on.
just taking pain relief is masking a problem. You are in pain because something is wrong, and it is vital to find out what it is, or at least rule out what it isn't that is causing your pain.
Best of Luck getting on top of whatever ails you.