UCO is based on futures. Hence there being contango, which is basically time erosion due to futures rolling over (contract months) continually if you hold a futures-based ETF for a while.
But yesterday was weird, this is a 2x leveraged ETF and oil went up 3% yesterday so UCO should have gone up 6% but it didn't. Only went up 3%. Today, UCO was back to normal & went up 2x the % that oil went up.