This is most likely an Impermanent Loss issue. I'll try to give a brief description, but you should really read up on it - it's critical to understanding the risks of being a liquidity provider.
Basically, if the price of an asset in a LP changes outside the pool (like the exchange price), then an arbitrage situation occurs. This can cause the ratio of assets in the pool to change. As the pool is arbitraged (and ratio altered), the value of the liquidity is recalculated based on the new ratio of assets.
In even shorter, your deposits to an LP can lose value depending on how the market moves.
Finematics has a great explainer, and links to article, which is also a very good read on the subject.