Help why is my labour so slow?

I had a membrane sweep from 12pm 20/03/18. I instantly had period cramp that never left. Also the midwife told me I was 1cm dilated. 21/03/18 my mucus plug came out (12:30pm) . I started having contractions straight after. At 6:30pm I had woken up from a nap in so much pain. My contractions were every 3 minutes apart and were getting stronger every couple of hours. The midwife told me wait until tomorrow morning because I could still talk. They said they wouldn’t even check me until I was crying. So 7am 22/03/18 I woke up and started timing the contractions and by 8am I was crying in agony. So they said they would see me now. I was only 2cm. They gave me liquid morphine to cope with the pain and sent me home. How can I have been having contractions every 3 minutes for well over 24 hours and still only be 2cm?

My baby is back to back (posterior position) so I don’t know if that’s the cause behind the pain and drawn out labour.