Miscarriage or heavy period?

My period last period was extremely light but I didn’t think anything of it. I’m on the pill and this most recent period I’m on now was about 4 days late. This morning I woke up not feeling well, slight headache and cramping. I had to wake up early and to work, but I got off at around noon. Since I’ve been home I’ve been having cramping in my upper thighs and lower back. My period blood is very dark and full of chunk and drown things. I looked up the symptoms which I know is never a good idea but I am genuinely concerned that I may be having an early miscarriage. I haven’t be very adamant on taking my pill the same time everyday which is why pregnancy could be possible. If anyone has any other ways to know the difference please tell me. I will go to the doctors as soon as I can to make sure.