We had issues with my son when he was around 20 months, and had to get help as he was nearly expelled from nursery for it. We had brilliant help from our HV and parentplus, who tested him and found his speech was delayed. Does LO talk much or make any noises. My son couldn't get his feelings across so was using his teeth to get attention. One thing the nursery did which worked really well was put him in time out but not say a word to him. We think that when he was being put in time out and being told why, he was still getting attention for it, so after a few times of no attention, he got better.
It could be teeth coming through, I know my son bit down on everything he could find when he was teething (although his biting people didn't start til much older) or it could be that your LO is trying to communicate but can't.
I know yours is a lot younger than mine was but it might help, and maybe a chat with your HV next time, they can offer some support / answers?
Good luck! I know it probably seems horrible now but it will pass in time!