Methergine after birth!!!!

I gave birth naturally to a healthy 8 on 6 oz baby 13 days ago. I stopped bleeding on day 9 and on day 12 started bleeding a lot again. I went to my OB who did an internal and said there's still a little blood left in my uterus. She gave me methergine to take every 6 hours for 1 day. I'm bleeding a lot and its supposed to stay that way until after the dose to get out what's left. Is this common? Why am I so nervous about taking this?! Can someone please calm my nerves and tell me they've taken this! (No horror stories please!)