It's not the last few days. It's the last few weeks because the price of STEEM compared to STEEM DOLLARS has risen quite significantly. Each SD only buys about a fraction of the STEEM it did a few weeks ago that means that your SD earnings are worth much less in STEEM POWER (1 STEEM=1SP). Hopefully the price of STEEM will fall back down again which will make it easier to power up.