Mmr and ranked???

Can anyone help me? (first time playing ranked) Heres what happened.

1. I played my placements and went 8/10 against golds and plat players
2. It didn't give me my rank after I completed placements.
3. After it said I was in Iron 3 and I had a mmr of platinum 4 (around 1760)
4. after a game my mmr dropps to gold 2?
5. next game I WON and my mmr tanks to bronze 1?
6. still playing well against plat players and carrying often and im in this elo??
7. Hallpppppp pliiiisssss????!?!!? Can someone fix this???!?!?