You should see your GP when you can.
In the meantime, you have three levels of escalation:
If it's "just" really bad cramps, and lots of red blood - it's probably either coil-related or a straightforward miscarriage.
(1) If you are in a lot of pain, ring 111 and/or go to your local out-of-hours clinic (I'm assuming you're in the UK and thus it's Saturday night for you)
The out of hours clinic should be able to do a pregnancy test for you.
(2) If the pain becomes so strong you can't cope, or if you are soaking through a maternity pad in about half an hour, get yourself to A&E quickly.
(3) If you start having pain in your shoulder, or excruciating pain on one side, call an ambulance. That could be an ectopic pregnancy, which is very serious and progresses quickly to being life-threatening. However, these are VERY RARE in comparison to a straightforward miscarriage (which feels utterly shit, but isn't likely to be life-threatening).
Good luck 