The yellow/orange line on your balance graph is your total unpaid and the blue line corresponds to your balance as listed above in the Wallet section. The "balance" is the total value of blocks (or shares) which you have mined, and have been confirmed and cleared through the exchange (the pool must sell them first before they can pay you). Naturally the "unpaid balance" is an estimated value of all the the "pending" blocks/shares which have not cleared the exchange yet. IT IS NORMAL FOR THIS AMOUNT TO FLUCTUATE; confirming and exchanging blocks takes time, and in this time period their value vs. your chosen wallet currency can change.
It is likely that what you observed while you were busy changing your payment currency was a drop in the value of your unconfirmed blocks/shares. If you need a concrete value of your account, I'd suggest using only the "balance" (blue line) indicator, which should only decrease in value when you are paid out.
The spike in your balance graph could be explained by an orphaned block. There isn't enough information in your screenshot to determine with certainty.
Okay I get the mentality of 'this must be some noob user error', honestly I do. Especially with me quote such low numbers, and this being a new account. I sold off most of my mining gear about a year ago, just doing a tiny bit here and there these days. But I am more than familiar with how these mining pools work.
So, I have decided to recreate the issue, with pictures!!
Here is the address before:
https://i.ibb.co/VM0hvCk/stageone.png
Now I mine to it for a couple of minutes with the currency set to LTC. Wait another 5 minutes or so and check the stats. Sure enough, it has been converted to LTC:
https://i.ibb.co/X84rd6z/stagetwo.png
Now I do the same thing to convert it back to BTC:
https://i.ibb.co/PQDZvNY/stagethree.png
Voila. I have now managed to lower the address's balance just by mining to it.
