Could I be demoted? :(

I got into Plat 5 a few weeks ago. So excited to have the border next season. I didn't want to risk anything so I was just going to let the season end and then play more ranked. I just got a message saying I will decay the day before the season ends... Poopy timing, right?

If I play a ranked game and lose now that I waited like 3 weeks, am I gonna be demoted and cry?

I know there is some grace period but don't know if it is by time or games or the lowest of ether or what. Would be useful to know in the future