Cramps, Tireness, Passing Blood Clots. Stopped Birth Control Pill, Had Sex

I think I may be miscarrying but I didn t know I was pregnant. I was having sex with my boyfriend regularly and using Microgynon birth control . I attended hospital for a pre op assessment as I am having knee surgery and the nurses told me that I had to stop using the pill that day for the operation to go ahead. So I stopped it about 28 days ago, the day after having sex and my period came about 3 days ago. My period normally lasts 3 days max and is very light, passes with no pain, only an increase in appetite but for the last 3 days I have been experiencing excruciating cramp and I have been very tired. I normally pass one or two little clots of dark blood but today and yesterday I have passed no less than 15 very large clots. The period blood I normally have is red but slightly gunge like and sticky, this time the blood looks exactly like if you were to bleed after cutting your arm (just normal blood). What should I do?