Low MMR = High LP loss

So ever since I've had a HUGE loss streak that tanked me from above average MMR in Gold III to below average in Gold IV, I've been losing ~20-21 LP and gaining ~14-15 every game. This is expected, obviously, but what's confusing me is that I seem to still be getting placed against Gold II-Is, whereas my team tends to be a 3-4 premade at my own rank. Can anyone explain what's going on here, or is it just bad luck?