I thought I understood how they work, but apparently I don’t? Here’s why. As I recall my opponent had played Helya and also shuffled at least 2 plagues into my deck through other means on other turns. I was playing druid. The turn before I conceded after misplaying due to the plagues not working as I thought they did, my cards cost 2 more, so my coin cost 2. I had drawn no plagues on the prior turn, but I had drawn 2 plagues in total on turns before this, as I recall. Only 1 of them was Frost tho as far as I remember so firstly, I would have thought they ought to cost 1 more, rather than 2 more. Secondly, in all my other games against Plague DK, after playing a card, the rest of my cards were back to the normal cost, This time tho, the cost didn’t change even after playing 2 cards, let alone one. Is this what is supposed to happen? Then, after I ended my turn, my cards were back to the normal cost. I have no idea why this is what happened. Please explain if possible. Thanks.
Frost plague is next card you play
I know. This doesn’t explain what I said. I know reading can be tedious but nice try!
Tbh I read it but kinda skimmed not sure why it’d cost 2 if your sure only one was a frost plague
I know you skimmed it 
I mean I was thinking maybe he played a stomper. I wasn’t really paying much attention. I reckon that might be it.
Did you or your opponent perhaps have a Razorscale in play?
https://hearthstone.fandom.com/wiki/Razorscale
Cards can’t cost less than (2).
That doesn’t fully explain everything you describe, but it would perhaps be a start.
If you played the game with an addon that records games, then please post a link to the replay. That will allow us to look into much greater detail at what happened, and whether this is a bug or an intended effect.
It wasn’t Razorscale as almost all my cards were 2 higher than normal in cost. I wasn’t using an addon as the pc I was using is slow so it can’t handle addons. Thanks for your input tho.